Except Retain Pattern

In many instances its desirable to traverse to only those elements that have not been seen in a previous step. Specific use cases are:

• “Who are my friends friends that are not already my friends?”
• “What is liked by the people that like the same things as me that I don’t already like?”

The solution to these types of problems is called the except pattern. Its opposite is the retain pattern—only traverse to those vertices that have been seen in a previous step.

gremlin> g.v(1).outE.inV
==>v[2]
==>v[3]
==>v[4]
gremlin> g.v(1).outE.inV.outE.inV
==>v[5]
==>v[3]

Both the first outE.inV and the second emit v[3]. To ensure that v[3] is not traversed to on the second step, its necessary to save the results seen after the first outE.inV. This pattern is so common, that there are high-level pipes called aggregate, except, and retain.

gremlin> x = [] as Set
gremlin> g.v(1).outE.inV.aggregate(x).outE.inV.except(x)
==>v[5]

gremlin> x = [] as Set
gremlin> g.v(1).outE.inV.aggregate(x).outE.inV.retain(x)
==>v[3]