DB::Table - An interface a database table & columns.

SYNOPSIS

use DB::Table;

my $table = DB::Table->open($dbh, $tableName);

my @primaryKeys = $table->primaryKeys(); # Primary Key(s) of table
my @fieldNames  = $table->fields();      # All other fields
my @foreignKeys = $table->foreignKeys(); # Which fields reference other tables?

my $field = $tabe->field('user_id');      # Get Field Data
print "Type: " . $field->{'type'};        # varchar? int?
print "Description: " . $field->{'desc'}; # User Description of Field.

DESCRIPTION

DB::Table is designed to be an abstrated description of a database table, such as which columns a table has, the type/length/etc. of a columnm and any descriptions of the table name/column names etc.

It tries to figure this information out by interrogating the database directly (when the table is open()ed) but it this module can also be subclassed, so that you can provide all this information yourself.

METHODS

Class Methods

my $table = DB::Table->open($dbh, [$tableName | $tableStructure]);

If you specify a table name (string) then DB::Table will create and return an object which represents all the information about the table specified by the string $tableName. This table must exist in the database that $dbh is currently connected to.

This method interrogates the database and attempts to make as much information available as what the database can provide. If the database supports comments on tables/columns, they are used as descriptions such that the internal name of a column can be 'user_id' while it can be presented to the user as 'User ID' (as an example).

You may also, however, specify your own $tableStructure (a hash-ref which describes the table) instead of having it pulled out of the database directly. This firstly allows one to subclass DB::Table, so that (as an example) one can have a DB::Table::User module who's open method can supply a pre-built table-structure to DB::Table's open method, but also allows for customising the representation of a given database table without modifying the database itself.

See the "SUBCLASSING" section below for more information on the way in which this can be done.

Object Methods

my $name = $table->componentName();

If this module is used as part of a MCV-type framework, then this method can be used to get at a version of the table-name that is all lower-case except for the first character. Its so that you can use it to derive the name of perl modules that handle the different aspects of the of the Controll/Model/View, for example, you may have a controller, called MyApp::Controller::[Table_name] who's job is to interact with the Model layer (of which this module can form a part).

If you are not using this module in an MCV-type framework, then you can just ignore this method

my @fieldNames = $table->fields();

This method returns an array (of strings) of the names of all fields (columns) in this table, except for the primary keys. To get the list of primary keys see the "primaryKeys" method below.

my @summaryFields = $table->summaryFields();

If you have supplied your own tableStructure, you can provide a list of "summaryFields" which can be a subset of the full list of fields, so that (as an example) if your application is displaying a list of users, you might not want to display everything about every user, but rather just display their name & date-of-birth.

If no summaryFields are defined, however, then this method acts the same as the fields method.

my @primaryKeys = $table->primaryKeys();

This method returns a list of the names of field(s) which are primary keys.

my @foreignKeys = $table->foreignKeys();

This method returns the list of field\column names in this table that refer to other tables.

my $field = $table->field($fieldName);

This method returns a hash reference structure which represents all of the properties the individual field specified by $fieldName

The elements of this hash are as follows (* means required):
name*      => scalar(string) : The name of the field/column
type*      => scalar(string) : The SQL data type of the field/column
length*    => scalar(int)    : The size of the field in bytes
nullable   => scalar(boolean): If this field/column can store NULL values
desc       => scalar(string) : The human description of this field/column
read       => scalar(boolean): Indicate if this column/field can be read *** NOT YET USED ***
write      => scalar(boolean): Indicate if this column/field can be modified *** NOT YET USED ***
constraint => scalar(string) : The SQL constraint associated with the field/column
                               for example, '(character_length(username) >= 5)' will evaluate
                               the query against the database before inserting/updating.
                               This is useful because, although PostgreSQL has built-in constraints,
                               I havent found a nice way to trap which field/value caused the failure,
                               and has the added advantage that databases that dont support
                               constraints but do support CASE..END can also make use of this.
validate   => {regex  => scalar(string) : In addition to the constraint field,
                                          data can also be validated using regular expressions,
                                          eg:  '^[\w\.]+\@[\w\.]+\.(com|net|org)$'
                                          could be used to validate that an email address
                                          belongs to the .com, .net or .org domain.
               error  => scalar(string)}: If validation fails, then this error message will be used.

The constraint field is prefered over using the validate regular expression, because (provided the database supports them in the first place) they will still apply regardless of the means in which the database is accessed. If the database does not support CASE...END statements, however, then this can be used as a fall-back.

my $tableName = $table->name();

This method provides access to the underlying table name.

my $desc = $table->desc();

The return value of the desc() method actually returns an array-reference. The first element being the singular description, to be used in the context of 1 row, while the second element is the plural of the first.

Tables have 2 descriptions: The first being the singular, the second being the plural of. For example, a table called 'software_users' could have a singular description of 'User' while the plural could be 'Users'. As this (and other) information is best stored in the database itself (in the form of comments) the comment should be 'User,Users' (ie, singular/plural seperated by a comma).

my $row = $table->constructRow($hashRef);

This method can be used to construct an instance of a DB::Table::Row object. If there happens to be a module called DB::Table::Row::[ current table name] then that module is used instead of the vanilla DB::Table::Row module. This allows the table to interact with subclasses of DB::Table::Row, so that you can customise it.

See DB::Table::Row for more info.

my $row = $table->getRowByPKey($id);

This method will return a DB::Table::Row object which represents the row identified by the primary key $id. If there happens to be a module called DB::Table::Row::[ current table name] then that module is used instead of the vanilla DB::Table::Row module. This allows the table to interact with subclasses of DB::Table::Row, so that you can customise it.

See DB::Table::Row for more info.

my $row = $table->getRowsByPKey($id);

This is the same as getRowByPKey but it provides the context of pluarlity, so using this method indicates that you're expecting more than 1 row.

my @rows = $table->getRowsByFKey($fkeyName);

If the specified $fkeyName is a foreign key which references a field in another table, then the rows belonging to the referenced table are returned where the refered to field contains the value of the specified $fkeyName. If there happens to be a module called DB::Table::Row::[ current table name] then that module is used instead of the vanilla DB::Table::Row module. This allows the table to interact with subclasses of DB::Table::Row, so that you can customise it.

my @rows = $table->searchRowsByString($string);

All rows in the specified table which have a column which matches the specified string are returned by this function.

TODO: Accept some sort of structure which defines a where clause, ie:
{ columnName => { matchType => '=', value => $string } }

my $foreignTable = $table->fkeyTable($foreignKey);

A Table object which represents the table referenced by the specified foreign key is returned. If there happens to be a module called DB::Table::[ foreign table name] then that module is used instead of the vanilla DB::Table module. This allows the table to interact with subclasses of DB::Table, so that you can customise it on a per-table basis.

my @tables = $table->referingTables([$keyField]);

With no arguments, a list of table objects which have a foreign key which refers to a key in $table. If a $keyField is specified, then only tables which refer to $keyField in $table are returned. If there happens to be a module called DB::Table::[ foreign table name] then that module is used instead of the vanilla DB::Table module. This allows the table to interact with subclasses of DB::Table, so that you can customise it on a per-table basis.

SUBCLASSING

If you wish to subclass this module, you may specify your own data-structure in your own open() method, and pass this as a parameter to this class's open() method. For example, consider a table which has a list of CPU's:

CREATE TABLE processor_types
(
  id serial NOT NULL,
  p_type varchar(255) NOT NULL UNIQUE,
  PRIMARY KEY (id),
);
COMMENT ON TABLE processor_types IS 'Processor Type,Processor Types';
COMMENT ON COLUMN processor_types.id IS 'Processor Type ID';
COMMENT ON COLUMN processor_types.p_type IS 'Processor Type';

CREATE TABLE servers
(
  id serial NOT NULL,
  ...
  processor_type_id int4 NOT NULL REFERENCES processor_types(id),
  PRIMARY KEY (id)
);
COMMENT ON TABLE servers IS 'Device,Device List';
...
COMMENT ON COLUMN servers.processor_type_id IS 'Processor Type';

You could then create a perl module called DB::Table::Processor_types, and a module called DB::Table::Servers as follows:

package DB::Table::Processor_types;
use DB::Table;
our @ISA = (qw(DB::Table));

sub open
{
  my $ref   = shift;
  my $class = ref($ref) || $ref;

  my $dbh = shift || confess ("Usage: $class->open(\$dbh)");

  my $tableData = {
        'pkeyFields' => [
                          'id'
                        ],
        'fields' => [
                      'p_type'
                    ],
        'tableDesc' => [
                         'Processor Type',
                         'Processor Types'
                       ],
        'tableName' => 'processor_types',
        'fkeyFields' => [],
        'rkeys' => [
                     {
                       'table' => 'servers',
                       'pkey' => 'id',
                       'field' => 'processor_type_id'
                     }
                   ],
        'field' => {
                     'p_type' => {
                                      'name' => 'processor',
                                      'length' => 255,
                                      'nullable' => '0',
                                      'desc' => 'Processor Type',
                                      'read' => 1,
                                      'type' => 'character varying',
                                      'constraint' => '(p_type != \'\')',
                                      'write' => 1
                                    },
                     'id' => {
                               'name' => 'id',
                               'length' => 4,
                               'nullable' => '0',
                               'desc' => 'Processor Type ID',
                               'read' => 1,
                               'pkey' => 1,
                               'type' => 'integer',
                               'constraint' => undef,
                             }
                   }
      };
  return $class->SUPER::open($dbh, $tableData);
}
1;

package DB::Table::Servers;

use Carp qw(cluck);
use DB::Table;
our @ISA = (qw(DB::Table));

sub open
{
    my $ref   = shift;
    my $class = ref($ref) || $ref;

    my $dbh = shift || confess ("Usage: $class->open(\$dbh)");
    my $tableData = {
        'pkeyFields' => [
                          'id'
                        ],
        'fields' => [
                      ...,
                      'processor_type_id',
                      ...
                    ],
        'tableDesc' => [
                         'Device',
                         'Device List'
                       ],
        'tableName' => 'servers',
        'fkeyFields' => [
                          'processor_type_id',
                        ],
        'field' => {
                     'id' => {
                               'name' => 'id',
                               'length' => 4,
                               'nullable' => '0',
                               'desc' => 'Device ID',
                               'read' => 1,
                               'pkey' => 1,
                               'type' => 'integer',
                               'constraint' => undef,
                               'validate' => {
                                               'regex' => '^.*$',
                                               'error' => ''
                                             }
                             },
                     'processor_type_id' => {
                                              'name' => 'processor_type_id',
                                              'fkey' => {
                                                          'table' => 'processor_types',
                                                          'field' => 'id'
                                                        },
                                              'length' => 4,
                                              'nullable' => '0',
                                              'desc' => 'Processor Type',
                                              'read' => 1,
                                              'type' => 'integer',
                                              'constraint' => '(processor_type_id > 0)',
                                              'write' => 1
                                            }
                   }
      };
  return $class->SUPER::open($dbh, $tableData);
}

1;

AUTHOR

Bradley Kite <bradley-cpan@kitefamily.co.uk>

If you wish to email me, then please remove the '-cpan' part of my email address as anything addressed to 'bradley-cpan' is assumed to be spam and is not read.

SEE ALSO

DB::Table::Row, DBI, perl