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    }

    unless ($pk_changed)
    {
        foreach my $new_pk ( $p{new}->primary_key )
        {
            my $old_col = eval { $p{old}->column( $new_pk->name ) };

            next if $new_pk->former_name && $changed_name{ $new_pk->former_name };

            unless ( $old_col && $old_col->is_primary_key )
            {
                push @sql, $self->alter_primary_key_sql( new => $p{new},
                                                         old => $p{old} );

                last;
            }
        }
    }

    my $alter_attributes;
    foreach my $new_att ( $p{new}->attributes )
    {
        unless ( $p{old}->has_attribute( attribute => $new_att, case_sensitive => 1 ) )
        {
            $alter_attributes = 1;

            push @sql, $self->alter_table_attributes_sql( new => $p{new},
                                                          old => $p{old},
                                                        );

            last;
        }
    }

    unless ($alter_attributes)
    {
        foreach my $old_att ( $p{old}->attributes )
        {
            unless ( $p{new}->has_attribute( attribute => $old_att, case_sensitive => 1 ) )
            {
                $alter_attributes = 1;

                push @sql, $self->alter_table_attributes_sql( new => $p{new},
                                                              old => $p{old},
                                                            );

                last;
            }
        }
    }

    return @sql;
}


sub _virtual
{
    my $self = shift;

    my $sub = (caller(1))[3];
    Alzabo::Exception::VirtualMethod->throw( error =>
                                             "$sub is a virtual method and must be subclassed in " . ref $self );
}

__END__

=head1 NAME

Alzabo::RDBMSRules - Base class for Alzabo RDBMS rulesets

=head1 SYNOPSIS

  use Alzabo::RDBMSRules;

  my $rules = Alzabo::RDBMSRules( rules => 'MySQL' );

=head1 DESCRIPTION

This class is the base class for all C<Alzabo::RDBMSRules> modules.
To instantiate a subclass call this class's C<new()> method.  See the
L<SUBCLASSING Alzabo::RDBMSRules> section for information on how to
make a ruleset for the RDBMS of your choice.

=head1 METHODS

=head2 available

A list of names representing the available C<Alzabo::RDBMSRules>
subclasses.  Any one of these names would be appropriate as the
"rdbms" parameter for the L<C<< Alzabo::RDBMSRules->new()
>>|Alzabo::RDBMSRules/new> method.

=head2 new

The constructor always accepts one parameter, "rdbms", which is the
name of the RDBMS to be used.

Some subclasses may accept additional values.

The constructor returns a new C<Alzabo::RDBMSRules> object of the
appropriate subclass.

Throws: L<C<Alzabo::Exception::Eval>|Alzabo::Exceptions>

=head2 schema_sql (C<Alzabo::Create::Schema> object)

Returns a list of SQL statements which would create the given schema.

=head2 index_sql (C<Alzabo::Create::Index> object)

Returns a list of SQL statements to create the specified index.

=head2 drop_table_sql (C<Alzabo::Create::Table> object)

Returns a list of SQL statements to drop the specified table.

=head2 drop_index_sql (C<Alzabo::Create::Index> object)

Returns a list of SQL statements to drop the specified index.