docs: def add_rel(self, source_node_id, target_node_id, rel):
          """Add a relationship between nodes.
  
          Args:
              source_node_id: Node Id for the source node.
              target_node_id: Node Id for the target node.
              rel: Name of the relationship 'contains'
          """
  
          # Add the relationship
          n1_ref = self.graph_db.get_indexed_node('Node', 'node_id', source_node_id)
          n2_ref = self.graph_db.get_indexed_node('Node', 'node_id', target_node_id)
  
          # Sanity check
          if not n1_ref or not n2_ref:
              print 'Cannot add relationship between unfound nodes: %s --> %s' % (source_node_id, target_node_id)
              return
          path = neo4j.Path(n1_ref, rel, n2_ref)
          path.get_or_create(self.graph_db)