= ActiveRecord Constraints

This plugin currently implements constraints for PostgreSQL only.  It
should provide a structure for a more abstract implementation.

Currently it implements foreign key constraints, unique
constraints and check constraints.  null and not null constraints
would be easy to add but they may collide with preexisting Active
Record code.

=== Examples

First the easy examples:

Unique Constraint:

    class CreateFoos < ActiveRecord::Migration
      def self.up
	create_table :foos do |t|
	  # name will have a "unique" constraint
	  t.string :name, :null => false, :unique => true
	end
      end
    end

Trivial foreign key constraint:

    class CreateFoos < ActiveRecord::Migration
      def self.up
	create_table :foos do |t|
	  # bar_id will now be a foreign key constraint column id in table bar
	  t.integer :bar_id, :null => false, :reference => true
	end
      end
    end

This is actually a short hand for:

    class CreateFoos < ActiveRecord::Migration
      def self.up
	create_table :foos do |t|
	  # bar_id will now be a foreign key constraint column id in table bar
	  t.integer :bar_id, :null => false, :reference => true,
	      :table_name => :bars, :foreign_key => :id
	end
      end
    end

If the constraint can not be done this easily, there are also unique,
reference, and check methods added to
ActiveRecord::ConnectionAdapters::TableDefinition.  So, for example:

    class CreateFoos < ActiveRecord::Migration
      def self.up
	create_table :foos do |t|
	  t.string :name1, :null => false
	  t.string :name2, :null => false
	  t.string :name3, :null => false
	  t.unique [ :name1, :name2, :name3 ]
	end
      end
    end

Or perhaps:

    class CreateFoos < ActiveRecord::Migration
      def self.up
	create_table :foos do |t|
	  t.integer :field1, :null => false
	  t.integer :field2, :null => false
	  t.integer :field3, :null => false
	  t.reference [ :field1, :field2, :field3 ],
	      :table_name => :bars,
	      :foreign_key => [ :bar_field1, :bar_field2, :bar_field3 ]
	end
      end
    end

Thats the front half.  The back half is catching the exceptions during
a save.  For example, if we have:

    class CreateFoos < ActiveRecord::Migration
      def self.up
	create_table :foos do |t|
	  # name will have a "unique" constraint
	  t.string :name, :null => false, :unique => true
	end
      end
    end

And then if we do:

  foo = Foo.new()
  foo.save

The save will throw an exception but the semantics of foo.save is to
return false if the constraints (validations) fail.  To keep this API,
the exception is caught and parsed trying to do what the standard
Rails constraints do.  In the above example, foo.errors.on(:name) will
be set to "can't be blank".

Contraints may also be named.  This allows the rescue to call a
specific method by the same name as the constraint.

=== Help with testing and fixtures

To work with the new fixtures, you must patch Rails:

    module ActiveRecord
      module ConnectionAdapters
	class PostgreSQLAdapter
	  def disable_referential_integrity(&block)
	    transaction {
	      begin
		execute "SET CONSTRAINTS ALL DEFERRED"
		yield
	      ensure
		execute "SET CONSTRAINTS ALL IMMEDIATE"
	      end
	    }
	  end
	end
      end
    end

Setting the schema_format to :sql is also a good idea.  In
environment.rb add:

  config.active_record.schema_format = :sql

And then you must make all the foreign key constraints deferrable.
So, the above example becomes:

    class CreateFoos < ActiveRecord::Migration
      def self.up
	create_table :foos do |t|
	  # bar_id will now be a foreign key constraint column id in table bar
	  t.integer :bar_id, :null => false, :reference => true,
	      :deferrable => true
	end
      end
    end

I like to have my foreign keys cascade on delete so now we have:

    class CreateFoos < ActiveRecord::Migration
      def self.up
	create_table :foos do |t|
	  # bar_id will now be a foreign key constraint on column id in table bar
	  t.integer :bar_id, :null => false, :reference => true,
	      :deferrable => true, :delete => :cascade
	end
      end
    end

But really I hate typing all that so, it now becomes:

    class CreateFoos < ActiveRecord::Migration
      def self.up
	create_table :foos do |t|
	  # bar_id will now be a foreign key constraint column id in table bar
	  t.fk :bar_id
	end
      end
    end

_fk_ is currently a silly stupid thing that needs to be fleshed out
more but the intent to make a "foreign key" something that Rails will
grok.

=== Future Directions

All the work code is in
ActiveRecord::ConnectionAdapters::Constraints.  For other data base
engines, this module 

Copyright (c) 2009 Perry Smith

This file is part of activerecord_constraints.

activerecord_constraints is free software: you can redistribute it
and/or modify it under the terms of the GNU General Public License as
published by the Free Software Foundation, either version 3 of the
License, or (at your option) any later version.

activerecord_constraints is distributed in the hope that it will be
useful, but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
General Public License for more details.

You should have received a copy of the GNU General Public License
along with activerecord_constraints.  If not, see
<http://www.gnu.org/licenses/>.