Ensure casting by decimal attribute when querying

Related 34cc301f03aea2e579d6687a9ea9782afc1089a0.

`QueryAttribute#value_for_database` calls only `type.serialize`, and
`Decimal#serialize` is a no-op unlike other attribute types.

Whether or not `serialize` will invoke `cast` is undefined in our test
cases, but it actually does not work properly unless it does so for now.

1 files changed, 4 insertions, 0 deletions

diff --git a/activemodel/lib/active_model/type/decimal.rb b/activemodel/lib/active_model/type/decimal.rb
index e8ee18c00e..b37dad1c41 100644
--- a/activemodel/lib/active_model/type/decimal.rb
+++ b/activemodel/lib/active_model/type/decimal.rb
@@ -12,6 +12,10 @@ module ActiveModel
         :decimal
       end
 
+      def serialize(value)
+        cast(value)
+      end
+
       def type_cast_for_schema(value)
         value.to_s.inspect
       end