If you have a class with STI where the descendant declares a new attribute:

class A

...

 

class B < A

field :secondary_description, type: String

 

And  you have a has many

class Something

...

has_many :as

accepts_nested_attributes_for :as

 

And then you create a form, upon creation, if you do

something = Something.new(allowed_params)

You get an error:

Attempted to set a value for 'secondary_description' which is not allowed on the model A

{{}}

This worked fine in Mongoid 6.0, it seems like the instantiation of the has_manys from params is not creating an in-memory object of type B, but instead, is creating an object of type A{{}}