Speed up homogeneous AR lists / reduce allocations

activerecord/lib/active_record/core.rb

@@ -344,6 +344,28 @@ def init_with(coder)
       self
     end
 
+    ##
+    # Initializer used for instantiating objects that have been read from the
+    # database.  +attributes+ should be an attributes object, and unlike the
+    # `initialize` method, no assignment calls are made per attribute.
+    #
+    # :nodoc:
+    def init_from_db(attributes)
+      init_internals
+
+      @new_record = false
+      @attributes = attributes
+
+      self.class.define_attribute_methods
+
+      yield self if block_given?
+
+      _run_find_callbacks
+      _run_initialize_callbacks
+
+      self
+    end
+
     ##
     # :method: clone
     # Identical to Ruby's clone method.  This is a "shallow" copy.  Be warned that your attributes are not copied.

activerecord/lib/active_record/persistence.rb

@@ -67,8 +67,18 @@ def create!(attributes = nil, &block)
       # how this "single-table" inheritance mapping is implemented.
       def instantiate(attributes, column_types = {}, &block)
         klass = discriminate_class_for_record(attributes)
+        instantiate_instance_of(klass, attributes, column_types, &block)
+      end
+
+      # Given a class, an attributes hash, +instantiate_instance_of+ returns a
+      # new instance of the class. Accepts only keys as strings.
+      #
+      # This is private, don't call it. :)
+      #
+      # :nodoc:
+      def instantiate_instance_of(klass, attributes, column_types = {}, &block)
         attributes = klass.attributes_builder.build_from_database(attributes, column_types)
-        klass.allocate.init_with("attributes" => attributes, "new_record" => false, &block)
+        klass.allocate.init_from_db(attributes, &block)
       end
 
       # Updates an object (or multiple objects) and saves it to the database, if validations pass.

activerecord/lib/active_record/querying.rb

@@ -49,7 +49,12 @@ def find_by_sql(sql, binds = [], preparable: nil, &block)
       }
 
       message_bus.instrument("instantiation.active_record", payload) do
-        result_set.map { |record| instantiate(record, column_types, &block) }
+        if result_set.includes_column?(inheritance_column)
+          result_set.map { |record| instantiate(record, column_types, &block) }
+        else
+          # Instantiate a homogeneous set
+          result_set.map { |record| instantiate_instance_of(self, record, column_types, &block) }
+        end
       end
     end
 

activerecord/lib/active_record/result.rb

@@ -43,6 +43,11 @@ def initialize(columns, rows, column_types = {})
       @column_types = column_types
     end
 
+    # Returns true if this result set includes the column named +name+
+    def includes_column?(name)
+      @columns.include? name
+    end
+
     # Returns the number of elements in the rows array.
     def length
       @rows.length

activerecord/test/cases/result_test.rb

@@ -12,6 +12,11 @@ def result
       ])
     end
 
+    test "includes_column?" do
+      assert result.includes_column?("col_1")
+      assert_not result.includes_column?("foo")
+    end
+
     test "length" do
       assert_equal 3, result.length
     end