So, as always, only after posting the question I get to answer =)
The solution is:
Place UserJustNameSerializer
into its own file user_just_name_serializer.rb
and use each_serializer:
instead of serializer:
, as I'm trying to serialize an array, not a single object.
For those who google: If I use serializer: UserJustNameSerializer
on array, I get
NoMethodError (undefined method `read_attribute_for_serialization' for []:Array):