I'm not sure if it's a bug or intentional but there are complications around querying has_and_belongs_to_many relationships that are not covered here and really tripped me up

given a Task with the following 

has_and_belongs_to_many :assignees, class_name: "Person", inverse_of: :tasks

and a Person that 

 has_and_belongs_to_many :tasks, class_name: "Task", inverse_of: :assignees

Expected Behavior

as a user i expect to be able to query for tasks that have people as their assignees using Person objects with the following

Task.in(assignees: [my_person_object]).count

I expect this because when there is a belongs_to relationship i can say things like this without problem

PullRequest.in(person: [my_person_object]).count

Actual Behavior

Task.in(assignees: [my_person_object]).count

but that produces the following error 

Value does not define its BSON serialized type: #<Person:0x00000001142b04c0> (BSON::Error::UnserializableClass)

The only way I've found to make it work is to do the following:

Task.in(:assignee_ids.in => [my_person_object.id]).count

Comments

I don't believe that it is correct that some relationships you can make an .in query with an array of Mongoid::Document objects, but in some you have to know the underlying fields that mongoid creates (in this case assignee_ids) and that you have to query using ids.

I believe .in queries should behave consistently and support the same types of objects in the array passed to it regardless of the relationship type it's querying on (presuming it makes any sense to use .in in the first place)