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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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)