code: def createEdge(self, collectionName, _fromId, _toId, edgeAttributes, waitForSync = False) :
          
  
          if not _fromId :
              raise ValueError("Invalid _fromId: %s" % _fromId)
  
          if not _toId :
              raise ValueError("Invalid _toId: %s" % _toId)
  
          if collectionName not in self.definitions :
              raise KeyError("'%s' is not among the edge definitions" % collectionName)
  
          url = "%s/edge/%s" % (self.URL, collectionName)
          self.database[collectionName].validatePrivate("_from", _fromId)
          self.database[collectionName].validatePrivate("_to", _toId)
          
          ed = self.database[collectionName].createEdge()
          ed.set(edgeAttributes)
          ed.validate()
  
          payload = ed.getStore()
          payload.update({'_from' : _fromId, '_to' : _toId})
  
          r = self.connection.session.post(url, data = json.dumps(payload, default=str), params = {'waitForSync' : waitForSync})
          data = r.json()
          if r.status_code == 201 or r.status_code == 202 :
              return self.database[collectionName][data["edge"]["_key"]]
          # print "\ngraph 160, ", data, payload, _fromId
          raise CreationError("Unable to create edge, %s" % r.json()["errorMessage"], data)
docs: creates an edge between two documents