added missing imports in helper file

mediagoblin_libreplanet/lp_helper.py

@@ -17,6 +17,8 @@
 # You should have received a copy of the GNU Affero General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
+from mediagoblin.db.models import MediaEntry, Tag, MediaTag
+
 def media_entries_for_tag_slug_case_insensitive(dummy_db, tag_slug):
     return MediaEntry.query \
         .join(MediaEntry.tags_helper) \