title: rails_admin look up association by field other than name in edit
body: <p>I am using rails_admin gem for CRUD on admin backend. I see that in edit, rails_admin creates a search box for associated object by name. I need to search the associated object by other fields. For instance, I want to set ID of a user for an object, and I want to use email to search for the user, instead of name. I may have users with same names. I see that in list configuration, you can set searchable :field_name. I tried this in edit configuration, but it didn't work.</p>