I found the answer right here : undefined method `users_user_id_eq' for Ransack::Search
There is also a post here : weird error popped up with active admin that looks to be the same problem description.
It seems that Ransack (used by ActiveAdmin) do not like has many .. :through
relationships.
Example :
If you have a user.rb
that has many :roles, :through => :user_roles
This will give you the error : undefined method user_roles_id_eq
To fix (not really fix.. but avoid to get an error) the bug, edit your "ActiveAdmin.register User" file, and add : remove_filter :user_roles