NAME

Database::Join - Read-only combined view across two or more Database::Abstraction objects

VERSION

Version 0.001.0

SYNOPSIS

Basic two-database join

use Database::Join;

# Step 1: create each component database the normal way
my $customers = Database::Customers->new(directory => '/data');
my $loyalty   = Database::Loyalty->new(directory  => '/data');

# Step 2: combine them on the shared key column 'entry'
my $join = Database::Join->new(
    databases   => [ $customers, $loyalty ],
    join_column => 'entry',
);

# Step 3: query exactly as you would a single Database::Abstraction object
my $all_rows  = $join->selectall_arrayref();
my $vip_rows  = $join->selectall_arrayref(tier => 'gold');
my $one_row   = $join->fetchrow_hashref(entry => 'C001');
my $total     = $join->count();
my $col_names = $join->columns();

Hiding internal columns

my $join = Database::Join->new(
    databases      => [ $customers, $loyalty ],
    join_column    => 'entry',
    remove_columns => [ 'internal_id', 'audit_ts' ],
);
# 'internal_id' and 'audit_ts' never appear in results or columns()

join_map: when the key column has different names in each database

# $cities  (index 0) has a column called 'statecode' -- matches join_column
# $stnames (index 1) has a column called 'entry'     -- different name

my $join = Database::Join->new(
    databases   => [ $cities,  $stnames ],
    #                index 0   index 1
    join_column => 'statecode',
    join_map    => { 1 => 'entry' },  # index 1 calls its join key 'entry'
);

# All returned rows use 'statecode'; 'entry' is never exposed
my $rows = $join->selectall_arrayref();

filters: permanently restrict a database's visible rows

# Only show orders placed more than 60 days ago, without repeating
# the criterion on every query call.
my $join = Database::Join->new(
    databases   => [ $customers, $orders ],
    join_column => 'entry',
    filters     => { 1 => { age_days => { '>' => 60 } } },
);

my $rows = $join->selectall_arrayref();                 # all old orders
my $vip  = $join->selectall_arrayref(tier => 'gold');   # old + gold tier

Inner and outer join types

my $inner = Database::Join->new(
    databases   => [ $customers, $loyalty ],
    join_column => 'entry',
    join_type   => 'inner',   # only keys present in BOTH databases
);

my $outer = Database::Join->new(
    databases   => [ $customers, $loyalty ],
    join_column => 'entry',
    join_type   => 'outer',   # all keys from EITHER database
);

Building the view incrementally with add_database

my $join = Database::Join->new(
    databases   => [ $customers ],
    join_column => 'entry',
);

$join->add_database($loyalty)
     ->add_database($scores, remove_columns => ['raw_score']);

AUTOLOAD column shortcut

# Returns the 'name' value for entry 'C001' (scalar context)
my $name = $join->name(entry => 'C001');

# Returns all 'tier' values (list context)
my @tiers = $join->tier();

DESCRIPTION

Database::Join merges two or more Database::Abstraction objects into a single logical, read-only view. Each component database is queried independently through its own Database::Abstraction interface. The results are combined in Perl memory using a shared key column (join_column).

The module exposes the same read-only API as Database::Abstraction: selectall_arrayref, selectall_array, fetchrow_hashref, count, columns, schema, updated, set_logger, and the AUTOLOAD column shortcut. Callers do not need to know how many underlying databases are involved.

Think of it as a virtual database table that is assembled on demand from several real tables, one per component database.

Join semantics

The join_type parameter controls what happens when a particular key value exists in some component databases but not all:

left (the default)

All rows from the primary (first) database are returned. Columns from subsequent databases are included where a matching row is found, and simply absent from the hashref where there is no match. If you are familiar with SQL, this is a LEFT OUTER JOIN on the first table.
inner

Only rows whose join-column value is present in every component database are returned. This is equivalent to a SQL INNER JOIN.
outer

Every join-column value found in any component database is returned. Columns from databases that do not have that key value are absent from the merged row. This is a FULL OUTER JOIN.

Important override rule: whenever you pass a query criterion for a column that belongs to a secondary database, that database automatically acts as an inner-join partner for that query only -- regardless of join_type. This gives WHERE-clause semantics. For example, if you have a LEFT join but query tier => 'gold' on a secondary database, only rows whose secondary entry has tier = 'gold' are returned (rows with no secondary entry are excluded, just as a WHERE clause would exclude them).

Column ownership and routing

At construction time, Database::Join calls columns() on each component database and builds an internal index that maps every column name to the database that owns it.

When you pass criteria to a query method, each key-value pair is automatically routed to the right database. You never need to say which database a column belongs to.

The join_column is special: criteria on it are broadcast to all databases so that each database fetches only the relevant rows before the in-memory merge.

When the same non-join column name exists in more than one database, the last database in the databases array wins: its value overwrites earlier ones in merged rows.

LIMITATIONS

In-memory join only

All matching rows from every component database are fetched into memory before the merge. This is not suitable for very large result sets.
No chained builder or raw SQL

query() and execute() are not implemented. Use selectall_arrayref or fetchrow_hashref instead.
Single-column equi-join only

Joining on more than one column simultaneously, or on expressions, is not supported. When the join key has different names in different databases, use join_map to declare each database's local column name.
Sort order

Results are sorted by the join_column value only. Caller-specified ORDER BY is not propagated to the component databases.
count() fetches all rows

count() executes the full join and counts the resulting rows in Perl. It does not push a COUNT(*) query down to the databases.

COMMON PITFALLS

The join_column must exist in every component database

If even one database is missing the join key column, new() (or add_database()) will croak immediately. Use join_map when the column has a different local name in some databases.
Criteria on a removed column are silently dropped

If you call remove_column('tier') and later query selectall_arrayref(tier => 'gold'), the criterion is ignored (with a carp warning) and all rows are returned. Always pass criteria before removing columns, or restructure your code to avoid this.
You cannot remove the join column

$join->remove_column($join->join_column) will croak. The join key is required for the merge to work.
Left join does not guarantee all columns are populated

Under a LEFT join, rows from the primary database that have no matching row in a secondary database will be returned with no keys from that secondary database. Accessing $row->{score} on such a row returns undef -- not zero, not an empty string. Always test defined $row-{score}> rather than just $row-{score}> when the secondary match is optional.
Filters act as inner-join partners

Any database that has a filters entry is promoted to an inner-join partner, regardless of join_type. A row whose join-key value does not appear in the filtered database's result is removed from the merged output entirely, not merely missing its secondary columns. This is intentional but can be surprising if you expected LEFT join semantics.
Criteria-merging replaces scalar filters

When both a base filter and a query criterion target the same column, and both are operator hashrefs (e.g. { '>' => 60 }), the operators are combined (AND semantics). But if the query criterion is a plain scalar (e.g. score => 75), it replaces the base filter for that column entirely -- the base filter is ignored for that query.
AUTOLOAD sees the full merged join when filters or join_map are active

When either filters or join_map is in effect, the AUTOLOAD shortcut ($join->columnname(...)) runs the full join query rather than delegating directly to the owning database. This is necessary for correctness but means the result respects all active filters and join-key translations, which may differ from what the owning database would return on its own.
Duplicate column names: last database wins

When two component databases each have a column called notes, the second database's value silently overwrites the first in every merged row. Use remove_columns (or remove_column) to drop the unwanted duplicate.

METHODS

new

SYNOPSIS

my $join = Database::Join->new(
    databases      => [ $db1, $db2 ],
    join_column    => 'entry',
    join_type      => 'left',
    join_map       => { 1 => 'local_col' },
    filters        => { 1 => { score => { '>' => 60 } } },
    remove_columns => [ 'email', 'internal_id' ],
    logger         => $log,
    i18n           => $locale,
);

DESCRIPTION

Constructs and returns a new Database::Join object.

Each element of databases must be an already-instantiated subclass of Database::Abstraction. The constructor calls columns() on every database to build an internal column-routing table and verifies that join_column (or its local alias from join_map) is present in each one.

Columns listed in remove_columns are hidden immediately: they do not appear in columns(), schema(), or any returned row hashref. This is equivalent to calling remove_column once per name after construction.

API SPECIFICATION

Input

databases      => { type => 'arrayref', required => 1 }
                  # One or more Database::Abstraction subclass objects.
                  #
                  # DOMAIN -- EP valid:   non-empty arrayref of blessed DA subclasses.
                  # DOMAIN -- EP invalid: scalar, hashref, or absent => croak.
                  # DOMAIN -- BVA size:   minimum 1 element; no documented upper bound.
                  # DOMAIN -- BVA elem:   each element must pass isa('Database::Abstraction').

join_column    => { type => 'string',   optional => 1, default => 'entry' }
                  # The column name shared by all databases (the join key).
                  #
                  # DOMAIN -- EP valid:   any non-empty string present in every component DA.
                  # DOMAIN -- EP invalid: column absent from any DA => croak join_col_missing.
                  # DOMAIN -- BVA:        empty string '' is treated as a column name and
                  #                       will croak if (as expected) it is absent from every DA.
                  # DOMAIN -- NOTE:       matching is case-sensitive and exact.

join_type      => { type => 'string',   optional => 1, default => 'left',
                    enum => ['inner', 'left', 'outer'] }
                  # Controls which keys appear in the result when not all
                  # databases share the same key values.
                  #
                  # DOMAIN -- EP valid:   exactly 'inner', 'left', or 'outer'.
                  # DOMAIN -- EP invalid: any other string including 'INNER', 'LEFT',
                  #                       'OUTER' (enum check is case-sensitive), 'cross',
                  #                       or '' => croak from validate_strict.

join_map       => { type => 'hashref',  optional => 1 }
                  # Zero-based database index => local column name.
                  # See the join_map section for full details.
                  #
                  # DOMAIN -- EP valid:   hashref values must be plain strings.
                  # DOMAIN -- EP invalid: reference value (hashref, arrayref, coderef, etc.)
                  #                       => croak; the guard prevents heap-address leakage.
                  # DOMAIN -- BVA:        out-of-range keys (beyond the databases array) are
                  #                       silently ignored.

filters        => { type => 'hashref',  optional => 1 }
                  # Zero-based database index => criteria hashref.
                  # Permanent row restrictions on individual databases.
                  # See the filters section for full details.

remove_columns => { type => 'arrayref', optional => 1 }
                  # Column names to hide from the merged view.
                  #
                  # DOMAIN -- EP valid:   arrayref of any strings; non-existent columns
                  #                       are silently ignored (idempotent).
                  # DOMAIN -- EP invalid: join_column itself => croak remove_join_col.
                  # DOMAIN -- BVA:        [] empty arrayref is a safe no-op.

logger         => { type => 'object',   optional => 1 }
                  # Logger object propagated to all component databases.

i18n           => { type => 'object',   optional => 1 }
                  # Localisation object with a translate($key, @args) method.

Output

A blessed Database::Join object.

EXAMPLE

# Customers database: entry | name | email
# Loyalty   database: entry | tier | points

my $join = Database::Join->new(
    databases      => [ $customers, $loyalty ],
    join_column    => 'entry',
    join_type      => 'inner',            # only customers who also have loyalty records
    remove_columns => [ 'email' ],        # hide PII from query results
    filters        => { 1 => { points => { '>' => 0 } } }, # ignore zero-point records
);

my $rows = $join->selectall_arrayref();
# Each row: { entry => ..., name => ..., tier => ..., points => ... }
# 'email' is absent. Zero-point loyalty records are excluded.

PSEUDOCODE

validate all parameters with validate_strict
croak if databases is empty
croak if any element of databases is not a Database::Abstraction subclass
bless the object with all fields initialised
call _build_col_index to map every column to its owning database
    and verify join_column presence in each database
for each column in remove_columns: call remove_column
return the new object

MESSAGES

error_no_databases     -- databases arrayref was empty
error_invalid_db       -- an element of databases is not a D::A subclass
error_join_col_missing -- join_column (or its join_map alias) not found in a database

join_map - joining on differently-named columns

By default every component database must have a column whose name matches join_column. If a database uses a different local name for the join key, declare the mapping with join_map.

join_map is a hashref. Each key is the zero-based position of a database in the databases array (0 = first, 1 = second, and so on). Each value is the name that that particular database uses for the join key.

Databases not listed in join_map are assumed to already have a column named join_column and need no entry.

Throughout the merged view the join key is always referred to by the name given in join_column. The local alias is never exposed in returned rows, in columns(), or in schema().

When do you need join_map?

You need join_map when you have two tables like:

cities table  : entry (the city name) | statecode
stnames table : entry (the state code) | state

Here you want to join cities.statecode to stnames.entry. You choose join_column => 'statecode' as the canonical name, but stnames calls that same concept entry, so you declare:

join_map => { 1 => 'entry' }  # stnames (index 1) calls it 'entry'

Example

#                        index 0     index 1
my @databases = (       $cities,    $stnames  );
#  join key column:    'statecode'  'entry'
#  join_column:        'statecode' (chosen canonical name)
#  stnames differs, so declare the alias:

my $join = Database::Join->new(
    databases   => \@databases,
    join_column => 'statecode',
    join_map    => { 1 => 'entry' },
);

my $rows = $join->selectall_arrayref();
# Each $row has keys: entry (city), statecode, state
# 'entry' from stnames is never exposed directly.

my $row = $join->fetchrow_hashref(statecode => 'CA');

Using add_database instead

If you build the join incrementally with add_database, pass join_column directly to that call instead of using join_map:

my $join = Database::Join->new(
    databases   => [ $cities ],
    join_column => 'statecode',
);
$join->add_database($stnames, join_column => 'entry');

This is exactly equivalent to the join_map form above.

filters - permanent per-database row filters

filters lets you restrict a component database to a subset of its rows permanently, without repeating the criterion on every query call.

Think of it as telling the join: "whenever you query this database, always add these extra conditions". Callers never need to specify the restriction themselves and can never accidentally omit it.

filters is a hashref. Each key is the zero-based position of a database in the databases array (same numbering as join_map). Each value is a criteria hashref in the same format as selectall_arrayref accepts.

Key-set semantics

A filtered database always acts as an inner-join partner, regardless of the join_type setting. Any join-key value that does not pass the filter is excluded from the merged output entirely -- not just missing its secondary columns. This ensures the filter genuinely restricts the view rather than simply hiding a few fields.

Criteria merging

When a query call also passes a criterion for a column that already has a base filter, the two constraints are combined:

When both the base filter value and the query criterion are operator hashrefs (e.g. { '>' => 60 } and { '<' => 365 }), their operators are merged: both constraints apply simultaneously (AND semantics).
When either value is a plain scalar, or the operators conflict, the query-time criterion wins and the base filter for that column is ignored for that one call.

Example -- only show orders placed more than 60 days ago

my $join = Database::Join->new(
    databases   => [ $customers, $orders ],
    join_column => 'entry',
    filters     => { 1 => { age_days => { '>' => 60 } } },
);

# Every query automatically sees only old orders
my $rows = $join->selectall_arrayref();

# Additional criteria layer on top -- gold tier AND old order
my $vip  = $join->selectall_arrayref(tier => 'gold');

# Range intersection: age_days > 60 AND age_days < 365
my $mid  = $join->selectall_arrayref(age_days => { '<' => 365 });

When using add_database, pass filter (singular) to set the base criteria for the new database:

$join->add_database($orders, filter => { age_days => { '>' => 60 } });

selectall_arrayref

SYNOPSIS

my $rows = $join->selectall_arrayref();
my $rows = $join->selectall_arrayref(tier  => 'gold');
my $rows = $join->selectall_arrayref(score => { '>' => 80 });
my $rows = $join->selectall_arrayref('C001');  # positional: entry => 'C001'

DESCRIPTION

Returns an arrayref of hashrefs representing the merged view of all component databases, optionally filtered by the given criteria.

Criteria for columns that live in different databases are routed automatically: each database is queried with only the criteria that apply to its own columns. The results are combined in memory using join_column.

Accepts the same criteria syntax as Database::Abstraction::selectall_arrayref. A single plain scalar argument is interpreted as the join_column value (equivalent to entry => 'C001' when join_column is 'entry').

API SPECIFICATION

Input

Calling conventions (in order of precedence):
  1. No arguments             -- returns all rows
  2. One plain scalar         -- shorthand for join_column => $scalar
  3. Key-value pairs or
     a criteria hashref       -- routed per-database

Values may be:
  Plain scalar                -- exact match
  Hashref of operators        -- e.g. { '>' => 80 }

Output

Arrayref of hashrefs; one hashref per qualifying merged row,
sorted ascending by join_column value.
Returns a reference to an empty array when no rows match.

EXAMPLE

# All rows from both databases
my $all = $join->selectall_arrayref();

# Only rows where the 'tier' column (from the loyalty database)
# equals 'gold' -- the criterion is routed to the right database
my $vip = $join->selectall_arrayref(tier => 'gold');

# Operator hashref: score > 80
my $high = $join->selectall_arrayref(score => { '>' => 80 });

# Access each merged row
for my $row (@{$vip}) {
    printf "%-10s tier=%-8s score=%d\n",
        $row->{entry}, $row->{tier}, $row->{score} // 0;
}

selectall_array

SYNOPSIS

my @rows = $join->selectall_array(tier => 'gold');

# Scalar context: only the first matching row
my $first = $join->selectall_array(entry => 'C001');

DESCRIPTION

In list context returns a list of merged hashrefs -- the same rows that selectall_arrayref would return, just as a flat list rather than an arrayref.

In scalar context returns only the first matching hashref (or undef if nothing matches).

API SPECIFICATION

Input

Same as selectall_arrayref.

Output

List context:   list of hashrefs (may be empty).
Scalar context: single hashref or undef.

EXAMPLE

my @all = $join->selectall_array();
print scalar @all, " rows\n";

# First gold-tier customer only
my $first_vip = $join->selectall_array(tier => 'gold');
print $first_vip->{name}, "\n" if defined $first_vip;

fetchrow_hashref

SYNOPSIS

my $row = $join->fetchrow_hashref(entry => 'C001');
my $row = $join->fetchrow_hashref('C001');   # positional shorthand

DESCRIPTION

Returns a single merged hashref for the first row matching the given criteria, or undef when nothing matches.

Equivalent to calling selectall_arrayref and taking only the first element. All the same criteria conventions apply.

API SPECIFICATION

Input

Same as selectall_arrayref.

Output

Hashref, or undef when no row matches.

EXAMPLE

my $row = $join->fetchrow_hashref(entry => 'C001');
if (defined $row) {
    print "Name: $row->{name}, Tier: $row->{tier}\n";
} else {
    print "No record for C001\n";
}

# Positional: works when join_column is 'entry'
my $row2 = $join->fetchrow_hashref('C001');

count

SYNOPSIS

my $total  = $join->count();
my $active = $join->count(tier => 'gold');

DESCRIPTION

Returns the number of merged rows that satisfy the given criteria.

The full join is performed and the resulting rows are counted in Perl; no COUNT(*) is pushed down to the component databases.

API SPECIFICATION

Input

Same criteria syntax as selectall_arrayref.

Output

Non-negative integer.

EXAMPLE

my $total   = $join->count();
my $gold    = $join->count(tier => 'gold');
my $high    = $join->count(score => { '>' => 90 });

printf "%d total, %d gold-tier, %d high-scorers\n",
    $total, $gold, $high;

columns

SYNOPSIS

my $cols = $join->columns();

DESCRIPTION

Returns an arrayref of all column names visible in the merged view, deduplicated and sorted alphabetically.

The join_column appears exactly once, even if it exists under different local names in some databases (see join_map). Columns that have been hidden with remove_column or remove_columns do not appear.

The result is memoised: repeated calls are cheap.

API SPECIFICATION

Input

None.

Output

Arrayref of column name strings, sorted alphabetically.

EXAMPLE

my $cols = $join->columns();
print join(', ', @{$cols}), "\n";
# e.g. "entry, name, score, tier"

schema

SYNOPSIS

my $schema = $join->schema();

DESCRIPTION

Returns a merged schema hashref for all visible columns across all component databases. Each key is a column name; each value is the schema metadata hashref returned by Database::Abstraction::schema() for that column (typically { type, nullable, default, pk }).

When the same column name appears in more than one database the last database's metadata is used. Columns hidden with remove_column are not included.

The result is memoised.

API SPECIFICATION

Input

None.

Output

Hashref: column_name => { type => ..., nullable => ..., default => ..., pk => ... }.

EXAMPLE

my $schema = $join->schema();
for my $col (sort keys %{$schema}) {
    my $info = $schema->{$col};
    printf "%-15s type=%-10s nullable=%s\n",
        $col, $info->{type}, $info->{nullable} ? 'yes' : 'no';
}

updated

SYNOPSIS

my $ts = $join->updated();

DESCRIPTION

Returns the Unix timestamp of the most recent modification across all component databases. This is the maximum of all individual updated() return values.

Use this to implement simple cache-invalidation logic: if updated() has advanced since your last snapshot, re-query.

API SPECIFICATION

Input

None.

Output

Unix timestamp (positive integer).

EXAMPLE

my $last_modified = $join->updated();
if ($last_modified > $my_cache_timestamp) {
    $my_cache = $join->selectall_arrayref();
    $my_cache_timestamp = $last_modified;
}

set_logger

SYNOPSIS

$join->set_logger($log);

DESCRIPTION

Attaches a new logger object to the join and propagates it to every component database. The logger is used for diagnostic output by all component databases.

API SPECIFICATION

Input

$log    Positional: a logger object (required).
        Must support whatever interface Database::Abstraction expects.

Output

Returns C<$self> for method chaining.

EXAMPLE

# Log::Any is used here as an example; any object that implements
# debug() and info() (or whichever methods your component databases
# call internally) works equally well.
use Log::Any qw($log);

my $join = Database::Join->new(databases => [$db1, $db2], join_column => 'entry');
$join->set_logger($log);
# $log is now used by $join and by $db1 and $db2

add_database

SYNOPSIS

# Positional: database object as first argument
$join->add_database($db);

# Named: equivalent to the above
$join->add_database(database => $db);

# With options (mixed positional + named)
$join->add_database($db, remove_columns => ['internal_id']);
$join->add_database($db, join_column    => 'local_key_name');
$join->add_database($db, filter         => { score => { '>' => 60 } });

# Chainable
$join->add_database($db1)->add_database($db2, remove_columns => ['notes']);

DESCRIPTION

Adds one more Database::Abstraction subclass object to the logical view and immediately updates the column-ownership index.

After the call, all query methods return rows that include columns from the newly added database, and criteria on those new columns are routed to it automatically.

When a column name in the new database already exists in an earlier database, the new database becomes the authoritative source for that column (last-database-wins, the same rule that applies at construction time).

The join-column must be present in the new database (or declared via join_column). The logger is propagated to the new database if one is set.

add_database is the runtime equivalent of listing the database in the databases array to new. The optional join_column parameter is equivalent to a join_map entry; the optional filter parameter is equivalent to a filters entry.

API SPECIFICATION

Input

database       => { type => 'object',   required => 1 }
                  # A Database::Abstraction subclass instance.
                  #
                  # DOMAIN -- EP valid:   blessed object that passes
                  #                       isa('Database::Abstraction').
                  # DOMAIN -- EP invalid: non-reference, unblessed ref, wrong class,
                  #                       or non-reference non-key scalar (the guard at
                  #                       the top of add_database rejects it with
                  #                       error_invalid_db before validate_strict runs).

join_column    => { type => 'string',   optional => 1 }
                  # The name of the join key in THIS new database,
                  # when it differs from the canonical join_column.
                  #
                  # DOMAIN -- EP valid:   any string that exists as a column in the
                  #                       new database.
                  # DOMAIN -- EP invalid: string absent from the new database's columns()
                  #                       => croak error_join_col_missing.

filter         => { type => 'hashref',  optional => 1 }
                  # Permanent criteria for this database only.
                  # Same format as selectall_arrayref.

remove_columns => { type => 'arrayref', optional => 1 }
                  # Column names from this database to hide.

Output

Returns C<$self> to support method chaining.

EXAMPLE

my $join = Database::Join->new(
    databases   => [ $customers ],
    join_column => 'entry',
);

# Add loyalty data; hide internal columns from it
$join->add_database($loyalty, remove_columns => ['audit_ts']);

# Add score data; only include rows with score > 60
$join->add_database($scores, filter => { score => { '>' => 60 } });

# Add a database whose join key has a different local name
$join->add_database($stnames, join_column => 'state_code');

# All three options combined, and chained
$join->add_database($db4,
    join_column    => 'ref_id',
    filter         => { active => 1 },
    remove_columns => ['legacy_col'],
);

PSEUDOCODE

determine the new database's index (length of current _dbs array)
extract the database object from positional or named argument
croak if it is not a Database::Abstraction subclass
register join_column alias in _join_map if different from canonical
register filter in _filters if provided
fetch column list from the new database
croak if the join key is missing from the new database
append the new database to _dbs and _db_cols
update _col_db: for each new column, point it at the new index
    (last-database-wins; skip removed columns and the local join alias)
invalidate _col_cache and _schema_cache
propagate logger if set
apply remove_columns if provided
return $self

MESSAGES

error_invalid_db       -- argument is not a Database::Abstraction subclass
error_join_col_missing -- join_column not found in the new database

remove_column

SYNOPSIS

$join->remove_column('email');

# Chainable
$join->remove_column('internal_id')->remove_column('audit_ts');

DESCRIPTION

Permanently hides a column from the merged view. After this call:

The column does not appear in columns() or schema().
Returned row hashrefs do not contain the column key.
Any query criterion that references the removed column is silently dropped (with a carp warning).

The join_column cannot be removed; attempting to do so will croak. Removing a column that does not exist in any database is silently ignored (the call is idempotent and safe). The columns() and schema() memoisation caches are cleared automatically.

API SPECIFICATION

Input

$col    Positional string: the column name to remove.

        DOMAIN -- EP valid:   any string; non-existent columns are silently
                              ignored (idempotent call, returns $self).
        DOMAIN -- EP invalid: join_column value => croak error_remove_join_col.
        DOMAIN -- BVA:        undef and '' are explicit no-ops (returns $self).
                              These are below the minimum meaningful string
                              length and are handled without any warning.

Output

Returns C<$self> to support method chaining.

EXAMPLE

# Hide private fields immediately after construction
my $join = Database::Join->new(
    databases   => [ $customers, $loyalty ],
    join_column => 'entry',
)->remove_column('email')
 ->remove_column('internal_notes');

# Verify they are gone
my $cols = $join->columns();
# 'email' and 'internal_notes' are absent

MESSAGES

error_remove_join_col -- attempt to remove the join_column itself

query

Not supported. Database::Join does not implement the chained query builder. Calling this method will always croak with an explanatory message.

Use selectall_arrayref, selectall_array, fetchrow_hashref, or count instead.

execute

Not supported. Raw SQL cannot span heterogeneous backends that may use different database engines. Calling this method will always croak.

Use selectall_arrayref or fetchrow_hashref to query the joined view.

AUTOLOAD - column shortcut

Calling an unknown method whose name matches a visible column name performs a column lookup across the merged view.

SYNOPSIS

# Scalar context: value from the first matching row
my $name  = $join->name(entry => 'C001');

# List context: values from every matching row
my @tiers = $join->tier();

# With a positional join-key argument (when join_column is 'entry')
my $score = $join->score('C001');

DESCRIPTION

AUTOLOAD routes the call to the appropriate component database by looking up the column name in the internal column-ownership index.

When either join_map or filters is active, AUTOLOAD performs a full join query instead of delegating directly to the owning database. This is necessary because:

With join_map, the owning database's primary key may differ from the canonical join key used in the call arguments.
With filters, bypassing the join would return rows that the filter is meant to exclude.

In list context, every matching merged row contributes one value to the returned list. In scalar context, only the first row's value is returned.

Calling a method whose name begins with _ (a private method) via AUTOLOAD will croak with a clear error message rather than being silently ignored.

EXAMPLE

# Lookup a single customer's name (scalar context)
my $name = $join->name('C001');   # 'C001' maps to entry => 'C001'
print "Name: $name\n";

# Get every tier value in the view (list context)
my @all_tiers = $join->tier();
my %freq;
$freq{$_}++ for @all_tiers;

# join_map active: AUTOLOAD runs a full join so the criteria are
# translated correctly between the canonical and local key names.
my @leesburg_states = sort $join->state('Leesburg');
# ['Florida', 'Virginia'] if Leesburg appears in two states

PSEUDOCODE

extract column name from $AUTOLOAD
return if DESTROY
croak if column name starts with '_' (private method guard)
croak if column name is not in _col_db (unknown column)
if join_map or filters are active:
    parse calling arguments using _parse_query_args
    call _joined_query to get all merged rows
    return map { $_->{col} } @rows  in list context
    return $rows[0]{col}            in scalar context
else:
    delegate directly to the owning database

MESSAGES

The following messages can be produced by Database::Join. All messages can be localised by supplying an i18n object to new.

error_no_databases

When: The databases arrayref passed to new is empty.

Fix: Pass at least one Database::Abstraction subclass object.
error_invalid_db

When: An element of the databases array (or the argument to add_database) is not an object, or is not a Database::Abstraction subclass.

Fix: Instantiate the component database with its own new method before passing it to Database::Join.
error_join_col_missing

When: The join key column (or its join_map alias) does not exist in one of the component databases.

Fix: Either add the column to the database, change join_column to a column that is present everywhere, or use join_map to declare the local alias for databases that call it something different.
error_remove_join_col

When: remove_column is called with the name of the join key column.

Fix: The join key is required for the merge to work and cannot be hidden. Remove a different column.
warn_unknown_column (carp)

When: A criterion is passed for a column that does not exist in any component database (or has been removed with remove_column).

Fix: Check the column name spelling. The criterion is ignored.
error_query_unsupported

When: query() is called on a Database::Join object.

Fix: Use selectall_arrayref, selectall_array, fetchrow_hashref, or count instead.
error_execute_unsupported

When: execute() is called on a Database::Join object.

Fix: Use the Perl-level query methods instead. Raw SQL cannot span heterogeneous database backends.

REPOSITORY

https://github.com/nigelhorne/Database-Join

SUPPORT

This module is provided as-is without any warranty.

SECURITY CONSIDERATIONS

Database::Join is a pure in-memory routing and merge layer. It never generates SQL strings, never opens files, and never calls system(), exec(), or eval(). The security properties described below are architectural guarantees, not run-time checks.

What Database::Join guarantees

Criteria partition isolation

Every criterion you pass to a query method is routed to exactly one component database (the one that owns that column), or to all databases when the criterion is on the join key column. A hostile value in a criterion for column name (owned by database A) will never reach database B.
Unknown columns are rejected before reaching any database

If a criterion column name is not present in any component database (or has been hidden with remove_column), Database::Join logs a carp warning and silently drops the criterion. No database receives the hostile key.
AUTOLOAD only accepts word-character column names

Perl's method dispatch extracts the column name via \w+, which matches only [A-Za-z0-9_]. Hostile method names with shell metacharacters, quotes, or spaces cannot reach the AUTOLOAD dispatch path. Private names (starting with _) are additionally blocked with an explicit croak.
No value sanitisation (by design)

Database::Join does not sanitise, HTML-encode, or validate the values in criteria hashrefs. Preventing SQL injection is the responsibility of the underlying Database::Abstraction objects (which use parameterised queries). Preventing XSS or header injection is the responsibility of the CGI or web layer that renders the output.
Taint-mode compatible

Database::Join contains no system(), exec(), backtick, open(PIPE), or eval STRING calls. It neither opens files nor constructs shell commands. The AUTOLOAD regex /::(\w+)$/ produces an untainted capture, so the column name used for dispatch is clean under -T. Criteria values are passed verbatim to component Database::Abstraction objects; those objects are responsible for handling tainted values at the SQL parameterisation layer.
Operator hashref aliasing

When the same join-key criterion (an operator hashref such as { '>' => 'A' }) is broadcast to multiple component databases, all of them receive a reference to the same hashref. A malicious component database that mutates the hashref's contents could affect what subsequent databases receive. Component databases are assumed to be trusted.

What the caller is responsible for

Sanitise values before building criteria

DJ passes criterion values verbatim to component databases. If your application accepts user-supplied filter values (e.g. from a CGI query string), those values must be validated or sanitised by your application before being passed to DJ.
Restrict which columns the caller can filter on

Any column in columns() can be used as a filter criterion. If a column should not be filterable by end users (e.g. an internal status flag), hide it with remove_column so that queries on it are silently dropped.
Do not expose the joined view directly to user-supplied criteria

DJ is not a firewall. It faithfully routes user input to component databases. Wrap DJ calls in a thin service layer that whitelists the permitted criterion columns and validates their values.

API SPECIFICATION (security surface)

Input accepted by all query methods and passed through DJ to component databases:

Criterion values:
    type: scalar string | operator hashref { OP => scalar }
    validation: NONE (DJ trusts the caller; component DA is responsible)
    max size: unconstrained (OOM risk on very large values)

Column name keys in criteria:
    type: string
    validation: must be present in _col_db (else carp + drop)
    character set: any Perl string (including control chars); DJ does
                   not impose a character-set restriction on criteria KEYS

AUTOLOAD method-name-as-column:
    type: \w+ (enforced by Perl regex /::(\w+)$/)
    validation: must not start with '_'; must be in _col_db

FORMAL SPECIFICATION

Z calculus schemas for the key invariants and operations. Unicode is used throughout this section as required by Z notation.

─── Database_Join ─────────────────────────────────────────────────
dbs        : seq DATABASE_ABSTRACTION
join_col   : NAME
join_type  : {left, inner, outer}
join_map   : ℕ ⇸ NAME
filters    : ℕ ⇸ CRITERIA
col_db     : NAME ⇸ ℕ
removed    : ℙ NAME
───────────────────────────────────────────────────────────────────
#dbs ≥ 1
dom join_map ⊆ 0 ‥ (#dbs - 1)
dom filters  ⊆ 0 ‥ (#dbs - 1)
dom col_db   = (⋃ { i : 0 ‥ #dbs-1 • ran((dbs i).columns) }) \ removed
join_col ∉ removed
∀ i : 0 ‥ #dbs-1 •
    local_jc(i) = if i ∈ dom join_map then join_map(i) else join_col
∀ i : 0 ‥ #dbs-1 •
    local_jc(i) ∈ ran((dbs i).columns)

─── Init ──────────────────────────────────────────────────────────
ΔDatabase_Join
dbs?       : seq DATABASE_ABSTRACTION
join_col?  : NAME
join_type? : {left, inner, outer}
join_map?  : ℕ ⇸ NAME
filters?   : ℕ ⇸ CRITERIA
removed?   : ℙ NAME
───────────────────────────────────────────────────────────────────
#dbs? ≥ 1
dbs'      = dbs?
join_col' = join_col?
join_type'= join_type?
join_map' = join_map?
filters'  = filters?
col_db'   = buildColIndex(dbs?, join_col?, join_map?)
removed'  = removed?

─── SelectAllArrayref ─────────────────────────────────────────────
ΞDatabase_Join        -- state unchanged
criteria? : CRITERIA
result!   : seq MERGED_ROW
───────────────────────────────────────────────────────────────────
∀ c : dom criteria? • c ∈ dom col_db ∪ {join_col}
result! = joinedQuery(criteria?)
result! is sorted ascending by join_col value

─── AddDatabase ───────────────────────────────────────────────────
ΔDatabase_Join
db?         : DATABASE_ABSTRACTION
local_jc?   : NAME   -- optional; defaults to join_col
filter?     : CRITERIA   -- optional
remove?     : ℙ NAME     -- optional
───────────────────────────────────────────────────────────────────
db?.isa('Database::Abstraction')
local_jc? ∈ ran(db?.columns)
dbs'      = dbs ^ ⟨db?⟩
col_db'   = col_db ⊕ { c ↦ #dbs | c ∈ ran(db?.columns) \ {local_jc?} \ removed }
filters'  = if filter? ≠ ∅ then filters ⊕ {#dbs ↦ filter?} else filters
join_map' = if local_jc? ≠ join_col
            then join_map ⊕ {#dbs ↦ local_jc?}
            else join_map
removed'  = removed ∪ remove?

─── RemoveColumn ──────────────────────────────────────────────────
ΔDatabase_Join
col? : NAME
───────────────────────────────────────────────────────────────────
col? ≠ join_col
removed'  = removed ∪ {col?}
col_db'   = col_db \ {col?}
join_map' = join_map
filters'  = filters
dbs'      = dbs

join_map

─── JoinMap ───────────────────────────────────────────────────────
join_map : ℕ ⇸ NAME
dbs      : seq DATABASE_ABSTRACTION
join_col : NAME
───────────────────────────────────────────────────────────────────
dom join_map ⊆ 0 ‥ (#dbs - 1)
∀ i : dom join_map • (join_map i) ∈ ran(dbs i).columns
∀ i : 0 ‥ (#dbs - 1) \ dom join_map •
    join_col ∈ ran(dbs i).columns

-- Resolution of the local join-key name for database i:
local_jc(i) == if i ∈ dom join_map then join_map(i) else join_col

-- The canonical name is always join_col; local_jc is never exposed.

SECURITY INVARIANTS

─── PartitionIsolation ─────────────────────────────────────────────
-- For every query call with criteria C and column col ≠ join_col:
∀ i : 0 ‥ #dbs-1 •
    i ≠ _col_db(col)  ⟹  col ∉ dom(per_db(i))

-- Unknown column is dropped before any database sees it:
col ∉ dom(_col_db) ∧ col ≠ join_col  ⟹
    (∀ i : 0 ‥ #dbs-1 • col ∉ dom(per_db(i)))

─── NoCodeExecution ────────────────────────────────────────────────
-- DJ contains no call to system(), exec(), open(PIPE), or eval().
-- Hostile criterion values therefore cannot achieve code execution
-- within the Database::Join layer.
∀ v : VALUE • _joined_query({col ↦ v}) ≠ ⊥ due to code injection

filters

─── Filters ─────────────────────────────────────────────────────
filters  : ℕ ⇸ CRITERIA
dbs      : seq DATABASE_ABSTRACTION
─────────────────────────────────────────────────────────────────
dom filters ⊆ 0 ‥ (#dbs - 1)

-- A filtered database i always contributes to key-set intersection.
-- For each query with criteria C:
effective_criteria(i, C) ==
    if i ∈ dom filters
    then merge_criteria(filters(i), partition(C, i))
    else partition(C, i)

-- Criteria merging (AND semantics for operator hashrefs):
merge_criteria(base, extra) ==
    { col : dom base ∪ dom extra •
        if col ∈ dom base ∩ dom extra
           ∧ base(col) ∈ HASHREF ∧ extra(col) ∈ HASHREF
        then col ↦ base(col) ∪ extra(col)   -- operator union
        else col ↦ (if col ∈ dom extra then extra(col) else base(col)) }

selectall_arrayref

selectall_arrayref : CRITERIA → seq MERGED_ROW
pre:  ∀ col : dom criteria • col ∈ dom self._col_db ∪ {self._join_col}
post: result = _joined_query(criteria)
      result is sorted ascending by join_col value

selectall_array

selectall_array : CRITERIA → seq MERGED_ROW | MERGED_ROW?
pre:  same as selectall_arrayref
post: wantarray  => result = @{ selectall_arrayref(criteria) }
      !wantarray => result = selectall_arrayref(criteria)[0]  (or undef)

fetchrow_hashref

fetchrow_hashref : CRITERIA → MERGED_ROW?
post: result = selectall_arrayref(criteria)[0]  (or undef if empty)

count

count : CRITERIA → ℕ
post: result = #selectall_arrayref(criteria)

columns

columns : → seq NAME
post: result = sort(
          (⋃ { i : 0 ‥ #dbs-1 • ran(dbs(i).columns) }
           \ dom removed_cols
           \ { local_jc(i) | i ∈ dom join_map ∧ local_jc(i) ≠ join_col })
      )

schema

schema : → NAME ⇸ SCHEMA_INFO
post: dom(result) = ran(columns())
      ∀ col : dom(result) •
          result(col) = (last database containing col).schema()(col)

updated

updated : → ℕ
post: result = max { i : 0 ‥ #dbs-1 • dbs(i).updated() }

remove_column

remove_column : NAME → Database_Join
pre:  col ≠ self._join_col
post: self'._removed_cols = self._removed_cols ∪ {col}
      self'._col_db       = self._col_db \ {col}
      self'._col_cache    = undef
      self'._schema_cache = undef

AUTOLOAD

AUTOLOAD : NAME × CRITERIA → VALUE | seq VALUE
pre:  col ∈ dom self._col_db
      col does not begin with '_'
post: let rows = _joined_query(criteria)
      wantarray  => result = { r : rows • r(col) }
      !wantarray => result = rows(0)(col)  (or undef if rows is empty)

AUTHOR

Nigel Horne, <njh@nigelhorne.com>

LICENSE AND COPYRIGHT

Usage is subject to the GPL2 licence terms. If you use it, please let me know.

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