As per the classic example for how to get an InvalidSetPolymorphicRelation:

class Face
include Mongoid::Document

has_one :left_eye, class_name: "Eye", as: :eyeable
has_one :right_eye, class_name: "Eye", as: :eyeable
end

class LeftEye
include Mongoid::Document

belongs_to :eyeable, polymorphic: true, inverse_of: :left_eye
end

class RightEye
include Mongoid::Document

belongs_to :eyeable, polymorphic: true, inverse_of: :right_eye
end

eye = LeftEye.new
face = Face.new
eye.eyeable = face # Raises Mongoid::Errors::InvalidSetPolymorphicRelation

If we add an extra class and specify which relationship we're referring to (inverse_of), it still blows up. Should this not be valid?