Performance optimization for ActiveRecord#column_defaults

Memoize the #column_defaults class property, as ActiveRecord does
for other properties in this module.

This change addresses slowness in ActiveRecord initialization
introduced starting in Rails 5.0. This method's performance has not
changed with Rails 5, but it is now called much more frequently than
before: every time an STI model is instantiated.

1 files changed, 2 insertions, 1 deletions

diff --git a/activerecord/lib/active_record/model_schema.rb b/activerecord/lib/active_record/model_schema.rb
index 54216caaaf..aa9087fae3 100644
--- a/activerecord/lib/active_record/model_schema.rb
+++ b/activerecord/lib/active_record/model_schema.rb
@@ -377,7 +377,7 @@ module ActiveRecord
       # default values when instantiating the Active Record object for this table.
       def column_defaults
         load_schema
-        _default_attributes.to_hash
+        @column_defaults ||= _default_attributes.to_hash
       end
 
       def _default_attributes # :nodoc:
@@ -466,6 +466,7 @@ module ActiveRecord
           @attribute_types = nil
           @content_columns = nil
           @default_attributes = nil
+          @column_defaults = nil
           @inheritance_column = nil unless defined?(@explicit_inheritance_column) && @explicit_inheritance_column
           @attributes_builder = nil
           @columns = nil