To whom it may concern, 

We have models like this:

class User
  has_many :clients
end

class Client
  has_and_belongs_to_many :tasks, counter_cache: true
end

And the issue is

user = User.create
user.clients.create(attributes)

Raising Error:
TypeError: no implicit conversion of Symbol into Integer

It was from here enumerable .
Because when creating contact I am not passing any task attributes.  At the same time counter_cache is trying to count tasks which is nil for this case.

It was working correctly for mongoid 6.4. 

Your fast reply is very appreciated as we are counting on version 7 to improve our server side performance. Many thanks!