Returns a list of tuples. The first element in each
        tuple is the coded value for the option that will
        appear in the URL query. The second element is the
        human-readable name for the option that will appear
        in the right sidebar.

def lookups(self, request, model_admin):
        """
        Returns a list of tuples. The first element in each
        tuple is the coded value for the option that will
        appear in the URL query. The second element is the
        human-readable name for the option that will appear
        in the right sidebar.
        """

        User = get_user_model()
        output = []
        for i in models.Model.objects.values('user__pk').distinct():
            pk = i['user__pk']

            if pk is not None:
                output.append([pk, User.objects.get(pk=pk).__str__])

        return output