Eagerly define attribute methods in production

The attribute methods for a model are currently defined lazily the first
time that model is instantiated, even when `config.eager_load` is true.
This means the first request to use each model incurs the cost, which
usually involves a database round trip to fetch the schema definition.

By defining the attribute methods for all models while the application
is booting, we move that work out of any individual request. When using
a forking web server, this also reduces the number of times the schema
definition is queried by doing it once in the parent process instead of
from each forked process during their first request.

activerecord/lib/active_record/railtie.rb

@@ -137,6 +137,14 @@ end_error
       end
     end
 
+    initializer "active_record.define_attribute_methods" do |app|
+      config.after_initialize do
+        ActiveSupport.on_load(:active_record) do
+          descendants.each(&:define_attribute_methods) if app.config.eager_load
+        end
+      end
+    end
+
     initializer "active_record.warn_on_records_fetched_greater_than" do
       if config.active_record.warn_on_records_fetched_greater_than
         ActiveSupport.on_load(:active_record) do

railties/test/application/configuration_test.rb

@@ -310,6 +310,53 @@ module ApplicationTests
       assert_equal %w(noop_email).to_set, PostsMailer.instance_variable_get(:@action_methods)
     end
 
+    test "does not eager load attribute methods in development" do
+      app_file "app/models/post.rb", <<-RUBY
+        class Post < ActiveRecord::Base
+        end
+      RUBY
+
+      app_file "config/initializers/active_record.rb", <<-RUBY
+        ActiveRecord::Base.establish_connection(adapter: "sqlite3", database: ":memory:")
+        ActiveRecord::Migration.verbose = false
+        ActiveRecord::Schema.define(version: 1) do
+          create_table :posts do |t|
+            t.string :title
+          end
+        end
+      RUBY
+
+      app "development"
+
+      assert_not_includes Post.instance_methods, :title
+    end
+
+    test "eager loads attribute methods in production" do
+      app_file "app/models/post.rb", <<-RUBY
+        class Post < ActiveRecord::Base
+        end
+      RUBY
+
+      app_file "config/initializers/active_record.rb", <<-RUBY
+        ActiveRecord::Base.establish_connection(adapter: "sqlite3", database: ":memory:")
+        ActiveRecord::Migration.verbose = false
+        ActiveRecord::Schema.define(version: 1) do
+          create_table :posts do |t|
+            t.string :title
+          end
+        end
+      RUBY
+
+      add_to_config <<-RUBY
+        config.eager_load = true
+        config.cache_classes = true
+      RUBY
+
+      app "production"
+
+      assert_includes Post.instance_methods, :title
+    end
+
     test "initialize an eager loaded, cache classes app" do
       add_to_config <<-RUBY
         config.eager_load = true