ActiveModel has a knob called include_root_in_json (documented in https://api.rubyonrails.org/classes/ActiveModel/Serializers/JSON.html) which controls the structure of the as_json method output. This knob is supported by Mongoid but its values are not being read correctly.

Test case: https://github.com/p-mongo/tests/blob/master/mongoid-app/lib/tasks/test_4865.rake

Output:

Mongoid: false
Foo: false
foo: false
Sub: false
sub: false

Mongoid -> true

Mongoid: true
Foo: false
foo: true
Sub: false
sub: true

Foo -> true

Mongoid: false
Foo: true
foo: false
Sub: true
sub: false

Sub -> true

Mongoid: false
Foo: false
foo: false
Sub: true
sub: false

The class level value is not influenced by the global Mongoid value, and the instance value is not influenced by the class level value.