Hi, guys
I have a small issue related with eager loading, I got error during eager loading.
Here are my models, CourseActivity inherit Activity, so I assume CourseActivity inherit 'belongs_to :group' relationship from Activity.

class Activity

  belongs_to :group

end

class CourseActivity < Activity

  has_one :assignment

end

class Group

  has_many :activities, class_name: 'Activity'

end

class Assignment

  belongs_to :course_activity

end

# failed with eager loader

CourseActivity.includes(:group, :assignment).to_a.count

# undefined method `assignment' for #<Group:0x007fbec6b2a020>

However if I add 'belongs_to :group' relationship in CourseActivity, eager loading works fine

class CourseActivity < Activity

  belongs_to :group

  has_one :assignment

end

# works perfect

CourseActivity.includes(:group, :assignment).to_a.count

I also enable preload_models in mongoid

#mongoid.yml

preload_models: true

I don't know that is caused by polymorphism belongs_to, since found on website "Eager loaded is supported on all relations with the exception of polymorphic belongs_to associations."