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= Foreign Key Migrations Foreign Key Migrations is a plugin that automatically generates foreign-key constraints when creating tables. It uses SQL-92 syntax and as such should be compatible with most databases that support foreign-key constraints. In the simplest case, the plugin assumes that if you have a column named +customer_id+ that you want a foreign-key constraint generated that references the +id+ column in the +customers+ table: create_table :orders do |t| t.column :customer_id, :integer, :null => false ... end If you have multiple columns referencing a table or for whatever reason, your column name isn't the same as the referenced table name, you can use the <code>:references</code> option: create_table :orders do |t| t.column :ordered_by_id, :integer, :null => false, :references => :customers ... end If you have a column with a name ending in +_id+ for which you do not wish a foreign-key to be generated, you can use <code>:references => nil</code>: create_table :orders do |t| t.column :external_id, :integer, :null => false, :references => nil ... end Sometimes you may (for legacy reasons) need to reference a primary key column that is named something other than +id+. In this case you can specify the name of the column: create_table :orders do |t| t.column :ordered_by_pk, :integer, :null => false, :references => [:customers, :pk] ... end You also have the option of specifying what to do on delete/update using <code>:on_delete</code>/<code>:on_update</code>, respectively to one of: <code>:cascade</code>; <code>:restrict</code>; and <code>:set_null</code>: create_table :orders do |t| t.column :customer_id, :integer, :on_delete => :set_null, :on_update => :cascade ... end If your database supports it (for example PostgreSQL) you can also mark the constraint as deferrable: create_table :orders do |t| t.column :customer_id, :integer, :deferrable => true ... end By convention, if a column is named +parent_id+ it will be treated as a circular reference to the table in which it is defined. Sometimes you may (for legacy reasons) need to name your primary key column such that it would be misinterpreted as a foreign-key (say for example if you named the primary key +order_id+). In this case you can manually create the primary key as follows: create_table :orders, :id => false do |t| ... t.primary_key :order_id, :references => nil end There is also a generator for creating foreign keys on a database that currently has none: ruby script/generate foreign_key_migration The plugin fully supports and understands the following active-record configuration properties: * <code>config.active_record.pluralize_table_names</code> * <code>config.active_record.table_name_prefix</code> * <code>config.active_record.table_name_suffix</code> === Dependencies * RedHill on Rails Core (redhillonrails_core). === See Also * Foreign Key Associations (foreign_key_associations). === License This plugin is copyright 2006 by RedHill Consulting, Pty. Ltd. and is released under the MIT license.