When you do has_many and belongs_to it creates a new property called tb_id and uses that to identify the relation.

My issue is found in two cases I guess many can relate.

MONGOID-4 You forgot to add has_many and belongs_to, or you didn't knew existed, and after a while when you added it all the custom relations you created don't automatically work or aren't recognized in the active record.

MONGOID-5 consider the following clases Question User, a question has a user that asked it and a user that answered it, using has_many and belongs_to only, if I call User.Questions will only bring the questions he asked.

My suggestion would be,

has_many: questions, field: answered_by, name: answers

That would allow me to get all the questions the user answered by doing User.answers instead of having to do
Question.where(:answered_by => user[:_id])

Sorry if I'm too n00b and this already is possible, but after centuries of research on the docs and forums I couldn't find how to do it.