Mongoid supports polymorphism for has_one, has_many, embeds_one, and embeds_many. Current implementation stores parent actual class name in the child collection. For example, for this association belongs_to :unit, polymorphic: true the class name is stored in unit_type attribute.

There is a demand to be able to infer the class name at runtime. See this article for example.

We should allow storing arbitrary string in *_type attributes, and let the application code decide what is an actual class of the association's target.