Only define attribute methods from schema cache

To define the attribute methods for a model, Active Record needs to know
the schema of the underlying table, which is usually achieved by making
a request to the database. This is undesirable behaviour while the app
is booting, for two reasons: it makes the boot process dependent on the
availability of the database, and it means every new process will make
one query for each table, which can cause issues for large applications.

However, if the application is using the schema cache dump feature, then
the schema cache already contains the necessary information, and we can
define the attribute methods without causing any extra database queries.

1 files changed, 63 insertions, 1 deletions

diff --git a/railties/test/application/configuration_test.rb b/railties/test/application/configuration_test.rb
index 9b01d42b1e..2220caf257 100644
--- a/railties/test/application/configuration_test.rb
+++ b/railties/test/application/configuration_test.rb
@@ -331,7 +331,7 @@ module ApplicationTests
       assert_not_includes Post.instance_methods, :title
     end
 
-    test "eager loads attribute methods in production" do
+    test "does not eager load attribute methods in production when the schema cache is empty" do
       app_file "app/models/post.rb", <<-RUBY
         class Post < ActiveRecord::Base
         end
@@ -354,9 +354,71 @@ module ApplicationTests
 
       app "production"
 
+      assert_not_includes Post.instance_methods, :title
+    end
+
+    test "eager loads attribute methods in production when the schema cache is populated" 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_file "config/initializers/schema_cache.rb", <<-RUBY
+        ActiveRecord::Base.connection.schema_cache.add("posts")
+      RUBY
+
+      app "production"
+
       assert_includes Post.instance_methods, :title
     end
 
+    test "does not attempt to eager load attribute methods for models that aren't connected" 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_file "app/models/comment.rb", <<-RUBY
+        class Comment < ActiveRecord::Base
+          establish_connection(adapter: "mysql2", database: "does_not_exist")
+        end
+      RUBY
+
+      assert_nothing_raised do
+        app "production"
+      end
+    end
+
     test "initialize an eager loaded, cache classes app" do
       add_to_config <<-RUBY
         config.eager_load = true