Mongoid::Association is poorly named. It is actually a mixin included in the Mongoid::Document class, which makes a document capable of being associated. (Mongoid::Associable would be a far better name, as the general practice is for mixins to end in -able suffix.)

The association metadata classes like HasOne, EmbedsMany, etc. do not inherit from Mongoid::Association. Instead, these include Mongoid::Association::Relatable mixin as their common base.

Therefore, where docs currently reference type Association (i.e. metadata) it is incorrect, we should replace it with Mongoid::Association::Relatable.