Using valueMap with match()

问题

I am calling JanusGraph remote, which returns ReferenceVertex by default. In order to retrieve properties as well, I use valueMap(), which works well for simple queries.

However, in my use case I need to build a join, which works well based on ReferenceVertex as follows:

// select t1.table2_ID, t2.table2_ID from table1 as t1 inner join table2 as t2 on t1.table2_ID = t2.table2_ID

GraphTraversal<?, ?> t1 = __.start().as("table1").out("relatesTo").hasLabel("table2").as("table2");
GraphTraversal<?, ?> t2 = g.V().hasLabel("table1").match(t1).select("table1", "table2");
List<?> l = t2.toList();

When adding valueMap to the traversal to retrieve the properties it fails. I want to include specific properties as follows:

// select t1.column1, t2.column2 from table1 as t1 inner join table2 as t2 on p.table2_ID = c.table2_ID

GraphTraversal<?, ?> t1 = __.start().as("table1").out("relatesTo").hasLabel("table2").valueMap(true, "column2").as("table2");
GraphTraversal<?, ?> t2 = g.V().hasLabel("table1").valueMap(true, "column1").match(t1).select("table1", "table2");
List<?> l = t2.toList();

-> java.util.HashMap cannot be cast to org.apache.tinkerpop.gremlin.structure.Element

Did I build the wrong traversal, or is this a limitation / bug in Tinkerpop?

Thank you

回答1:

You can just modulate the select() to apply valueMap() to each column you return. Here's an example using the modern toy graph that ships with TinkerPop:

gremlin> g.V().as('a').out().as('b').select('a','b').by(valueMap())
==>[a:[name:[marko],age:[29]],b:[name:[lop],lang:[java]]]
==>[a:[name:[marko],age:[29]],b:[name:[vadas],age:[27]]]
==>[a:[name:[marko],age:[29]],b:[name:[josh],age:[32]]]
==>[a:[name:[josh],age:[32]],b:[name:[ripple],lang:[java]]]
==>[a:[name:[josh],age:[32]],b:[name:[lop],lang:[java]]]
==>[a:[name:[peter],age:[35]],b:[name:[lop],lang:[java]]]

so in your case you would just do:

GraphTraversal<?, ?> t1 = __.start().as("table1").out("relatesTo").hasLabel("table2").as("table2");
GraphTraversal<?, ?> t2 = g.V().hasLabel("table1").match(t1).select("table1", "table2").by(__.valueMap());
List<?> l = t2.toList();

That said, unless you've abbreviated your code a bit for purpose of this question, I'm not sure I see the need for match() in this case. Seems like you could just simplify that to just:

List<?> l = g.V().hasLabel("table1").
              out("relatesTo").hasLabel("table2").as("table2").
              select("table1", "table2").
                by(__.valueMap()).toList();

来源：https://stackoverflow.com/questions/53621774/using-valuemap-with-match