I have Two model, Unit & Resident and following relationship.

class Unit 
  belongs_to :primary_resident, class_name: "Resident", inverse_of: :primary_in_units
end

class Resident
  has_many :primary_in_units, class_name: "Unit", inverse_of: :primary_resident, dependent: :nullify
end

When I am firing this command on rails console.
Unit.last.as_json(only: [:uuid, :name ], methods: :primary_resident)

Primary resident return only uuid and name that is keys specified for unit model.

After debugging, I found that mongoid serialize calling relation with all options we have passed which they should not.

  File: lib/mongoid/serializable.rb

   def serialize_attribute(attrs, name, names, options)   
      if relations.has_key?(name)
        value = send(name)
        attrs[name] = value ? value.serializable_hash(options) : nil 
      elsif names.include?(name) && !fields.has_key?(name)
        attrs[name] = read_attribute(name)
      elsif !attribute_missing?(name)
        attrs[name] = send(name)
      end 
   end

Here name is relationship in our case it is primary_resident , then it is passing same options to even relationship objects.

I know the correct way to do this is to use include: but using the above methods: option should definitely throw an error and does not return me primary_unit with same keys as specified for unit model?