Apache TinkerPop•3mo ago

select T.id + optional properties

I am trying to work out how to select vertex id and some optional properties. select().by does not work as it filters out not productive properties. Here is the sample graph I am testing this on.

TinkerGraph graph = TinkerGraph.open();
Vertex a = graph.addVertex(T.label, "A", "prop1", "aaaa");
Vertex b1 = graph.addVertex(T.label, "B", "prop2", "bbbb");
Vertex b2 = graph.addVertex(T.label, "B");
a.addEdge("ab", b1);
a.addEdge("ab", b2);

List<Map<String, Vertex>> result1 = graph.traversal().V().hasLabel("A").as("a")
        .out("ab").as("b")
        .<Vertex>select("a", "b").by("prop1").by("prop2")
        .toList();
for (Map<String, Vertex> objectObjectMap : result1) {
    System.out.println(objectObjectMap);
}
List<Map<Object, Object>> result2 = graph.traversal().V().hasLabel("A").as("a")
        .out("ab").as("b")
        .select("a", "b")
        .elementMap(T.id.getAccessor(), "prop1", "prop2")
        .toList();
for (Map<Object, Object> objectObjectMap : result2) {
    System.out.println(objectObjectMap);
}

TinkerGraph graph = TinkerGraph.open();
Vertex a = graph.addVertex(T.label, "A", "prop1", "aaaa");
Vertex b1 = graph.addVertex(T.label, "B", "prop2", "bbbb");
Vertex b2 = graph.addVertex(T.label, "B");
a.addEdge("ab", b1);
a.addEdge("ab", b2);

List<Map<String, Vertex>> result1 = graph.traversal().V().hasLabel("A").as("a")
        .out("ab").as("b")
        .<Vertex>select("a", "b").by("prop1").by("prop2")
        .toList();
for (Map<String, Vertex> objectObjectMap : result1) {
    System.out.println(objectObjectMap);
}
List<Map<Object, Object>> result2 = graph.traversal().V().hasLabel("A").as("a")
        .out("ab").as("b")
        .select("a", "b")
        .elementMap(T.id.getAccessor(), "prop1", "prop2")
        .toList();
for (Map<Object, Object> objectObjectMap : result2) {
    System.out.println(objectObjectMap);
}

The first gremlin produces one row as it filters out the 'B' with no 'prop2'

{a=aaaa, b=bbbb}

{a=aaaa, b=bbbb}

The second gremlin crashes with

java.lang.ClassCastException: class java.util.LinkedHashMap cannot be cast to class org.apache.tinkerpop.gremlin.structure.Elemen

java.lang.ClassCastException: class java.util.LinkedHashMap cannot be cast to class org.apache.tinkerpop.gremlin.structure.Elemen

I am looking to retrieve A.id, prop1, B.id, prop2 where prop1 and prop2 to are optional fields.

2 Replies

spmallette•3mo ago

there's a number of ways you might try to solve that. how about just using coalesce() in the by():

gremlin> g = TinkerFactory.createModern().traversal()
==>graphtraversalsource[tinkergraph[vertices:6 edges:6], standard]
gremlin> g.V().as('a').out().as('b').select('a','b').by('name').by('age')
==>[a:marko,b:27]
==>[a:marko,b:32]
gremlin> g.V().as('a').out().as('b').select('a','b').by('name').by(coalesce(values('age'), constant(-1)))
==>[a:marko,b:-1]
==>[a:marko,b:27]
==>[a:marko,b:32]
==>[a:josh,b:-1]
==>[a:josh,b:-1]
==>[a:peter,b:-1]

gremlin> g = TinkerFactory.createModern().traversal()
==>graphtraversalsource[tinkergraph[vertices:6 edges:6], standard]
gremlin> g.V().as('a').out().as('b').select('a','b').by('name').by('age')
==>[a:marko,b:27]
==>[a:marko,b:32]
gremlin> g.V().as('a').out().as('b').select('a','b').by('name').by(coalesce(values('age'), constant(-1)))
==>[a:marko,b:-1]
==>[a:marko,b:27]
==>[a:marko,b:32]
==>[a:josh,b:-1]
==>[a:josh,b:-1]
==>[a:peter,b:-1]

the second one fails because you did select(...).elementMap(...) which isn't possible because elementMap() can't work on the Map that select() produces.

pieterOP•3mo ago

Thanks, I'll try coalesce. This is what I ended up going with,

List<Map<String, Object>> result = graph.traversal().V().hasLabel("A").as("a")
        .out("ab").as("b")
        .select("a", "b")
        .by(__.elementMap("prop1"))
        .by(__.elementMap("prop2"))
        .toList();

List<Map<String, Object>> result = graph.traversal().V().hasLabel("A").as("a")
        .out("ab").as("b")
        .select("a", "b")
        .by(__.elementMap("prop1"))
        .by(__.elementMap("prop2"))
        .toList();

Thanks

Gaming

Programming

select T.id + optional properties