5 files changed, 43 insertions, 2 deletions
diff --git a/activerecord/lib/active_record/core.rb b/activerecord/lib/active_record/core.rb
index e1a0b2ecf8..9c9fc77a0d 100644
--- a/activerecord/lib/active_record/core.rb
+++ b/activerecord/lib/active_record/core.rb
@@ -350,6 +350,26 @@ module ActiveRecord
     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
+
+      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.
     # That means that modifying attributes of the clone will modify the original, since they will both point to the
diff --git a/activerecord/lib/active_record/persistence.rb b/activerecord/lib/active_record/persistence.rb
index a0d5f1ee9f..72d92ef504 100644
--- a/activerecord/lib/active_record/persistence.rb
+++ b/activerecord/lib/active_record/persistence.rb
@@ -67,8 +67,14 @@ module ActiveRecord
       # 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+ returns a new instance
+      # of the class. Accepts only keys as strings.
+      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.
diff --git a/activerecord/lib/active_record/querying.rb b/activerecord/lib/active_record/querying.rb
index d33d36ac02..c84f3d0fbb 100644
--- a/activerecord/lib/active_record/querying.rb
+++ b/activerecord/lib/active_record/querying.rb
@@ -49,7 +49,12 @@ module ActiveRecord
       }
 
       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
 
diff --git a/activerecord/lib/active_record/result.rb b/activerecord/lib/active_record/result.rb
index ffef229be2..3549e8a3fa 100644
--- a/activerecord/lib/active_record/result.rb
+++ b/activerecord/lib/active_record/result.rb
@@ -43,6 +43,11 @@ module ActiveRecord
       @column_types = column_types
     end
 
+    # Does this result set include 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
diff --git a/activerecord/test/cases/result_test.rb b/activerecord/test/cases/result_test.rb
index db52c108ac..68fcafb682 100644
--- a/activerecord/test/cases/result_test.rb
+++ b/activerecord/test/cases/result_test.rb
@@ -12,6 +12,11 @@ module ActiveRecord
       ])
     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