docs, `instantiate` expects `String` keys. [Rafal Piekarski & Yves Senn]

Closes #15122
Closes #15107

2 files changed, 15 insertions, 4 deletions

diff --git a/activerecord/lib/active_record/persistence.rb b/activerecord/lib/active_record/persistence.rb
index 13d7432773..b74e340b3e 100644
--- a/activerecord/lib/active_record/persistence.rb
+++ b/activerecord/lib/active_record/persistence.rb
@@ -37,7 +37,7 @@ module ActiveRecord
       end
 
       # Given an attributes hash, +instantiate+ returns a new instance of
-      # the appropriate class.
+      # the appropriate class. Accepts only keys as strings.
       #
       # For example, +Post.all+ may return Comments, Messages, and Emails
       # by storing the record's subclass in a +type+ attribute. By calling
@@ -46,10 +46,10 @@ module ActiveRecord
       #
       # See +ActiveRecord::Inheritance#discriminate_class_for_record+ to see
       # how this "single-table" inheritance mapping is implemented.
-      def instantiate(record, column_types = {})
-        klass = discriminate_class_for_record(record)
+      def instantiate(attributes, column_types = {})
+        klass = discriminate_class_for_record(attributes)
         column_types = klass.decorate_columns(column_types.dup)
-        klass.allocate.init_with('attributes' => record, 'column_types' => column_types)
+        klass.allocate.init_with('attributes' => attributes, 'column_types' => column_types)
       end
 
       private
diff --git a/activerecord/test/cases/persistence_test.rb b/activerecord/test/cases/persistence_test.rb
index 5d963098fb..bc5ccd0fe9 100644
--- a/activerecord/test/cases/persistence_test.rb
+++ b/activerecord/test/cases/persistence_test.rb
@@ -843,4 +843,15 @@ class PersistenceTest < ActiveRecord::TestCase
 
     assert_equal "Wright Glider", Aircraft.last.name
   end
+
+  def test_instantiate_creates_a_new_instance
+    post = Post.instantiate("title" => "appropriate documentation", "type" => "SpecialPost")
+    assert_equal "appropriate documentation", post.title
+    assert_instance_of SpecialPost, post
+
+    # body was not initialized
+    assert_raises ActiveModel::MissingAttributeError do
+      post.body
+    end
+  end
 end