[entities] fix previous_entity and next_entity methods since entry_of is an optional relation

entities.py

@@ -100,11 +100,17 @@
     # IPrevNext interface #####################################################
 
     def next_entity(self):
-        rql = ('Any B ORDERBY B ASC LIMIT 1 '
-               'WHERE B is BlogEntry, B entry_of BL, BL eid %(blog)s, '
-               'B eid > %(eid)s')
-        rset = self.req.execute(rql, {'blog': self.entry_of[0].eid, 'eid': self.eid})
+        if self.entry_of:
+            rql = ('Any B ORDERBY B ASC LIMIT 1 '
+                   'WHERE B is BlogEntry, B entry_of BL, BL eid %(blog)s, '
+                   'B eid > %(eid)s')
+            rset = self.req.execute(rql, {'blog': self.entry_of[0].eid, 'eid': self.eid})
+        else:
+            rql = ('Any B ORDERBY B ASC LIMIT 1 '
+                   'WHERE B is BlogEntry, B eid > %(eid)s')
+            rset = self.req.execute(rql, {'eid': self.eid})
+
         if rset:
             return rset.get_entity(0,0)
 
     def previous_entity(self):
@@ -107,11 +113,16 @@
         if rset:
             return rset.get_entity(0,0)
 
     def previous_entity(self):
-        rql = ('Any B ORDERBY B DESC LIMIT 1 '
-               'WHERE B is BlogEntry, B entry_of BL, BL eid %(blog)s, '
-               'B eid < %(eid)s')
-        rset = self.req.execute(rql, {'blog': self.entry_of[0].eid, 'eid': self.eid})
+        if self.entry_of:
+            rql = ('Any B ORDERBY B DESC LIMIT 1 '
+                   'WHERE B is BlogEntry, B entry_of BL, BL eid %(blog)s, '
+                   'B eid < %(eid)s')
+            rset = self.req.execute(rql, {'blog': self.entry_of[0].eid, 'eid': self.eid})
+        else:
+            rql = ('Any B ORDERBY B DESC LIMIT 1 '
+                   'WHERE B is BlogEntry, B eid < %(eid)s')
+            rset = self.req.execute(rql, {'eid': self.eid})
         if rset:
             return rset.get_entity(0,0)