collection_singular_ids in case of a has_many trough with source - ruby-on-rails

Question

I just have found a strange behavior of Active Record, where I am not sure if I am the problem or Active Record ;-).
I am using: Rails 4, Ruby 2.0
Here the models:
class User < ActiveRecord::Base
has_many :conversation_participants, dependent: :destroy
has_many :conversations, through: :conversation_participants
end
class ConversationParticipant < ActiveRecord::Base
belongs_to :user
belongs_to :conversation
end
class Conversation < ActiveRecord::Base
has_many :conversation_participants, dependent: :destroy
has_many :participants, through: :conversation_participants, source: :user
end
Pretty straight forward besides the has_many :participants, which refers to user over source:.
Normally, it is possible to call collection_singular_ids on an has_many associated object. Like in this case:
User.find(1).conversation_ids
This works perfectly, I get back an array of conversations ids. But from the other side:
Conversation.find(1).participant_ids
I alway getting back nil. When I go back to the standard for the has_many:
class Conversation < ActiveRecord::Base
has_many :conversation_participants, dependent: :destroy
has_many :users, through: :conversation_participants
end
Conversation.find(1).user_ids
will work fine.
When I look into the resulting queries, I see what the reason could be:
has_many :users creates:
SELECT "users".id FROM "users" INNER JOIN "conversation_participants" ON "users"."id" = "conversation_participants"."user_id" WHERE "conversation_participants"."conversation_id" = $1 [["conversation_id", 29]]
has_many :participants creates:
SELECT "users".* FROM "users" INNER JOIN "conversation_participants" ON "users"."id" = "conversation_participants"."user_id" WHERE "conversation_participants"."conversation_id" = $1 [["conversation_id", 29]]
in the participants case I get a SELECT "users".* instead SELECT "users".id
(using class_name: "User" has also not helped)
Bug or human error?