def lookup_allowed(self, lookup):
        allowed = super(admin.ModelAdmin, self).lookup_allowed(lookup)
        if not allowed:
            from django.db.models.sql.constants import LOOKUP_SEP, QUERY_TERMS 
        
            parts = lookup.split(LOOKUP_SEP)
    
            # Last term in lookup is a query term (__exact, __startswith etc)
            # This term can be ignored.
            if len(parts) > 1 and parts[-1] in QUERY_TERMS:
                parts.pop()
            clean_lookup = LOOKUP_SEP.join(parts)
            if clean_lookup == "medewerker__user_ptr":
                allowed = True

        return allowed