2 files changed, 11 insertions, 5 deletions
diff --git a/activerecord/lib/active_record/attribute_methods/primary_key.rb b/activerecord/lib/active_record/attribute_methods/primary_key.rb
index b9b2acff37..081aad434d 100644
--- a/activerecord/lib/active_record/attribute_methods/primary_key.rb
+++ b/activerecord/lib/active_record/attribute_methods/primary_key.rb
@@ -21,7 +21,7 @@ module ActiveRecord
       # Sets the primary key value.
       def id=(value)
         sync_with_transaction_state
-        write_attribute(self.class.primary_key, value) if self.class.primary_key
+        _write_attribute(self.class.primary_key, value) if self.class.primary_key
       end
 
       # Queries the primary key value.
diff --git a/activerecord/lib/active_record/attribute_methods/write.rb b/activerecord/lib/active_record/attribute_methods/write.rb
index 75c5a1a600..d7ddd35dff 100644
--- a/activerecord/lib/active_record/attribute_methods/write.rb
+++ b/activerecord/lib/active_record/attribute_methods/write.rb
@@ -17,7 +17,7 @@ module ActiveRecord
             generated_attribute_methods.module_eval <<-STR, __FILE__, __LINE__ + 1
               def __temp__#{safe_name}=(value)
                 name = ::ActiveRecord::AttributeMethods::AttrNames::ATTR_#{safe_name}
-                write_attribute(name, value)
+                _write_attribute(name, value)
               end
               alias_method #{(name + '=').inspect}, :__temp__#{safe_name}=
               undef_method :__temp__#{safe_name}=
@@ -36,8 +36,7 @@ module ActiveRecord
         end
 
         name = self.class.primary_key if name == "id".freeze && self.class.primary_key
-        @attributes.write_from_user(name, value)
-        value
+        _write_attribute(name, value)
       end
 
       def raw_write_attribute(attr_name, value) # :nodoc:
@@ -46,10 +45,17 @@ module ActiveRecord
         value
       end
 
+      # This method exists to avoid the expensive primary_key check internally, without
+      # breaking compatibility with the write_attribute API
+      def _write_attribute(attr_name, value) # :nodoc:
+        @attributes.write_from_user(attr_name.to_s, value)
+        value
+      end
+
       private
         # Handle *= for method_missing.
         def attribute=(attribute_name, value)
-          write_attribute(attribute_name, value)
+          _write_attribute(attribute_name, value)
         end
     end
   end