Generate add_index by default when giving type belongs_to or references

activerecord/CHANGELOG

@@ -1,5 +1,27 @@
 *Rails 3.1.0 (unreleased)*
 
+* When a model is generated add_index is added by default for belongs_to or references columns 
+
+  rails g model post user:belongs_to will generate the following:
+
+    class CreatePosts < ActiveRecord::Migration
+      def up
+        create_table :posts do |t|
+          t.belongs_to :user
+
+          t.timestamps
+        end
+
+        add_index :posts, :user_id
+      end
+
+      def down
+        drop_table :posts
+      end
+    end
+
+  [Santiago Pastorino]
+
 * Setting the id of a belongs_to object will update the reference to the
 object. [#2989 state:resolved]
 

activerecord/lib/rails/generators/active_record/model/templates/migration.rb

@@ -8,6 +8,10 @@ class <%= migration_class_name %> < ActiveRecord::Migration
       t.timestamps
 <% end  -%>
     end
+
+<% attributes.select  {|attr| attr.reference? }.each do |attribute| -%>
+    add_index :<%= table_name %>, :<%= attribute.name %>_id
+<% end  -%>
   end
 
   def down

railties/guides/source/getting_started.textile

@@ -811,6 +811,8 @@ class CreateComments < ActiveRecord::Migration
 
       t.timestamps
     end
+
+    add_index :comments, :post_id
   end
 
   def self.down
@@ -819,7 +821,7 @@ class CreateComments < ActiveRecord::Migration
 end
 </ruby>
 
-The +t.references+ line sets up a foreign key column for the association between the two models. Go ahead and run the migration:
+The +t.references+ line sets up a foreign key column for the association between the two models. And the +add_index+ line sets up an index for this association column. Go ahead and run the migration:
 
 <shell>
 $ rake db:migrate

railties/test/generators/scaffold_generator_test.rb

@@ -3,7 +3,7 @@ require 'rails/generators/rails/scaffold/scaffold_generator'
 
 class ScaffoldGeneratorTest < Rails::Generators::TestCase
   include GeneratorsTestHelper
-  arguments %w(product_line title:string price:integer)
+  arguments %w(product_line title:string product:belongs_to user:references)
 
   setup :copy_routes
 
@@ -14,7 +14,10 @@ class ScaffoldGeneratorTest < Rails::Generators::TestCase
     assert_file "app/models/product_line.rb", /class ProductLine < ActiveRecord::Base/
     assert_file "test/unit/product_line_test.rb", /class ProductLineTest < ActiveSupport::TestCase/
     assert_file "test/fixtures/product_lines.yml"
-    assert_migration "db/migrate/create_product_lines.rb"
+    assert_migration "db/migrate/create_product_lines.rb", /belongs_to :product/
+    assert_migration "db/migrate/create_product_lines.rb", /add_index :product_lines, :product_id/
+    assert_migration "db/migrate/create_product_lines.rb", /references :user/
+    assert_migration "db/migrate/create_product_lines.rb", /add_index :product_lines, :user_id/
 
     # Route
     assert_file "config/routes.rb" do |route|