if org_repo.scm_instance.alias != other_repo.scm_instance.alias:

            msg = 'compare of two different kind of remote repos not available'

            log.error(msg)

            h.flash(msg, category='error')

            return redirect(url('compare_home', repo_name=c.repo_name))

        c.cs_rev = self._get_ref_rev(other_repo, other_ref_type, other_ref_name)

        c.compare_home = False

        c.a_repo = org_repo

        c.a_ref_name = org_ref_name

        c.a_ref_type = org_ref_type

        c.update_msg_other = ""

        if org_scm_instance.alias == 'hg' and c.a_ref_name != 'ancestor':

            if c.cs_ref_type != 'branch':

                c.cs_branch_name = org_scm_instance.get_changeset(c.cs_ref_name).branch # use ref_type ?

            other_branch_name = c.a_ref_name

            if c.a_ref_type != 'branch':

            # candidates: descendants of old head that are on the right branch

            #             and not are the old head itself ...

            #             and nothing at all if old head is a descendent of target ref name

            if other_scm_instance._repo.revs('present(%s)::&%s', c.cs_ranges[-1].raw_id, other_branch_name):

                c.update_msg = _('This pull request has already been merged to %s.') % other_branch_name

            else: # look for children of PR head on source branch in org repo

            c.repository_forks = self.scm_model.get_forks(dbr)

            c.repository_pull_requests = self.scm_model.get_pull_requests(dbr)

            c.repository_following = self.scm_model.is_following_repo(

                                    c.repo_name, self.authuser.user_id)

    @staticmethod

        """

        Safe way to get changeset. If error occurs show error.

        """

        from kallithea.lib import helpers as h

        try:

            return repo.scm_instance.get_ref_revision(ref_type, ref_name)

        except EmptyRepositoryError as e:

            h.flash(h.literal(_('There are no changesets yet')),

                    category='error')

            raise webob.exc.HTTPNotFound()

        except ChangesetDoesNotExistError as e:

            h.flash(h.literal(_('Changeset not found')),

                    category='error')