Add a Computed field to a collection

I have 2 collection types

  1. post
  2. category

On the post model I have a relationship field that enables the selection for related categories.
On the category display(on the admin table) i want to add a column that will display a count of how many posts are related to that category.

I have tries using the lifecycles model but it seems that the afterFind hook is not working on admin requests.

How can i add this kind of field (i dont want it to be saved to the db because it is a dynamic field based on other collection)