Remove ActiveRecord::AttributeAssignment#assign_nested_parameter_attributes

The private `#assign_nested_parameter_attributes` was introduced in
[774ff18][] (Dec 15, 2011), moved to `ActiveRecord::AttributeAssignment`
in [ceb33f8][] (also Dec 15, 2011), then most recently modified in
[2606fb3][] (Jan 23, 2015) when `ActiveModel::AttributeAssignment` was
extracted.

Support for `accepts_nested_attributes_for` (introduced in [ec8f045][]
Feb 1, 2009) pre-dates those commits, and has evolved enough to cover
this behavior in other ways.

With its removal, Active Record's test suite still passes, so if it's a
crucial piece of code to retain, we should expand the suite to exercise
it.

[774ff18]: rails@774ff18
[ceb33f8]: rails@ceb33f8
[2606fb3]: rails@2606fb3
[ec8f045]: rails@ec8f045

Author: seanpdoyle

activerecord/lib/active_record/attribute_assignment.rb

@@ -4,29 +4,21 @@ module ActiveRecord
   module AttributeAssignment
     private
       def _assign_attributes(attributes)
-        multi_parameter_attributes = nested_parameter_attributes = nil
+        multi_parameter_attributes = nil
 
         attributes.each do |k, v|
           key = k.to_s
 
           if key.include?("(")
             (multi_parameter_attributes ||= {})[key] = v
-          elsif v.is_a?(Hash)
-            (nested_parameter_attributes ||= {})[key] = v
           else
             _assign_attribute(key, v)
           end
         end
 
-        assign_nested_parameter_attributes(nested_parameter_attributes) if nested_parameter_attributes
         assign_multiparameter_attributes(multi_parameter_attributes) if multi_parameter_attributes
       end
 
-      # Assign any deferred nested attributes after the base attributes have been set.
-      def assign_nested_parameter_attributes(pairs)
-        pairs.each { |k, v| _assign_attribute(k, v) }
-      end
-
       # Instantiates objects for all attribute classes that needs more than one constructor parameter. This is done
       # by calling new on the column type or aggregation type (through composed_of) object with these parameters.
       # So having the pairs written_on(1) = "2004", written_on(2) = "6", written_on(3) = "24", will instantiate