NAME

Alzabo::ForeignKey - Foreign key (relation) objects

SYNOPSIS

  use Alzabo::ForeignKey;

  foreach my $fk ($table->foreign_keys)
  {
      print $fk->cardinality;
  }

DESCRIPTION

A foreign key is an object defined by several properties. It represents a relationship from a column in one table to a column in another table. This relationship can be described via a sentence "There is a relationship from column baz in table foo to column Y in table bar. For every entry in column baz in table foo, there must X-Y corresponding entries in table bar. For every entry in column boz in table bar, there must be Y-Z corresponding entries in table foo." X, Y, and Z are the number 0, 1, or n.

The properties that make up a foreign key are:

table_from - The table that 'owns' the foreign key.

table_to - The table to which the relationship is made.

column_from - The column in the owning table that corresponds to some column in 'table_to'.

column_to - The column to which there is a correspondence.

min_max(_to, _from) - Legal values for this are 0..1, 0..n, 1..1, and 1..n (n..n relationships are handled specially). For the above mentioned relationship from foo.baz to bar.boz, if the min_max_to were 0..1, we could say, "For every entry in foo.baz, there may be 0 or 1 corresponding entries in bar.boz." If the min_max_from value were 1..n we would say that "for every entry in bar.boz, there must be 1 or more corresponding entries in foo.baz."

Cardinality is generated from the two min_max values. This is the max from to the max to. If min_max_from was 0..1 and min_max_to was 1..n then the cardinality of the relationship would be 1..n.

METHODS

• table_from

• table_to

• column_from

• column_to

  Returns the relevant object for the property.

• min_max_from, min_max_to

  Returns a two element array containing the two portions of the min_max value.

• cardinality

  Returns a two element array containing the two portions of the cardinality of the relationship.

AUTHOR

Dave Rolsky, <autarch@urth.org>