Improve performance of AR object instantiation

We introduced a performance hit by adding an additional iteration
through a model's attributes on creation. We don't actually need the
values from `Result` to be a hash, we can separate the columns and
values and zip them up ourself during the iteration that we have to do.

1 files changed, 25 insertions, 3 deletions

diff --git a/activerecord/lib/active_record/persistence.rb b/activerecord/lib/active_record/persistence.rb
index 755ff2b2f1..9d2c9d3b9c 100644
--- a/activerecord/lib/active_record/persistence.rb
+++ b/activerecord/lib/active_record/persistence.rb
@@ -65,19 +65,41 @@ module ActiveRecord
       # how this "single-table" inheritance mapping is implemented.
       def instantiate(attributes, column_types = {})
         klass = discriminate_class_for_record(attributes)
-        attributes = klass.attributes_builder.build_from_database(attributes, column_types)
-        klass.allocate.init_with('attributes' => attributes, 'new_record' => false)
+        klass.instantiate_pairs(attributes.keys, attributes.values, column_types)
+      end
+
+      def instantiate_pairs(columns, values, column_types = {}) # :nodoc:
+        attributes = attributes_builder.build_from_database_pairs(columns, values, column_types)
+        allocate.init_with('attributes' => attributes, 'new_record' => false)
+      end
+
+      def instantiate_result_set(result_set, column_types = {}) # :nodoc:
+        inheritance_column_index = inheritance_column && result_set.columns.find_index(inheritance_column)
+
+        result_set.each_pair.map do |columns, values|
+          inheritance_value = inheritance_column_index && values[inheritance_column_index]
+          klass = discriminate_class_for_value(inheritance_value)
+          klass.instantiate_pairs(columns, values, column_types)
+        end
       end
 
       private
         # Called by +instantiate+ to decide which class to use for a new
         # record instance.
         #
-        # See +ActiveRecord::Inheritance#discriminate_class_for_record+ for
+        # See +ActiveRecord::Inheritance#discriminate_class_for_value+ for
         # the single-table inheritance discriminator.
+        def discriminate_class_for_value(*)
+          self
+        end
+
         def discriminate_class_for_record(record)
           self
         end
+
+        def inheritance_column
+          nil
+        end
     end
 
     # Returns true if this object hasn't been saved yet -- that is, a record