Verify edge and create edge in the same instruction (gremlin python)

Question

My initial logic of checking if an edge is present and creating an edge needs to query. Im trying to verify and create an edge in one instruction.

This query does not seem to work

ipdb> prop = self._graph.V('pppp').outE('friend').hasId('testEdge').as_('e').inV()
             .hasId('dddd').select('e').
             coalesce(__.property('testedder', 1111).fold().unfold(), 
                __.V('dddd'). as_('to_a').V('pppp').addE('friend').to('to_a')).
             toList()

1) The first part of the coalesce - updating the property of Edges work fine

2) The second part of the coalesce is either not being called or not working. It is working as an independent query. Does 'as' not work in anonymous traversals?

PS: Im using AWS Neptune

Answer 1

You had the right idea but you needed some simplification. I'll try to do it in steps. First, whenever I see labelled steps, I try to see if there is a way to avoid using them. In this case, they can be factored out:

g.V('pppp').outE('friend').
  filter(hasId('testEdge').inV().hasId('dddd')).
  coalesce(__.property('testedder', 1111).fold().unfold(), 
           __.V('dddd'). as_('to_a').V('pppp').addE('friend').to('to_a'))

Readability of the traversal improves on those first two lines because the reader can immediately see that you want to find an edge given some criteria which was only implied by the step labeling approach. Next, I looked at coalesce(). As it stands, if filter() returns no edges then coalesce() will never get a chance to execute and that's why the second part of coalesce() never has an opportunity to work for you. So, let's clean that part up:

g.V(1).outE('knows').
  filter(hasId(6).inV().hasId(2)).
  fold().
  coalesce(unfold().property('testedder', 1111),
           V('dddd').as_('to_a').V('pppp').addE('friend').to('to_a'))

If it's not clear why the fold() and unfold() are where they are, you should check out my detailed explanation of the approach here. So, with fold() and unfold() where they should be, the coalesce() should now trigger both conditions depending on whether or not an edge passes the filter(). The first part of the coalesce() is fine, but the second could still use a bit of work as I'd again like to factor out the step labels if the aren't necessary:

g.V('pppp').outE('friend').
  filter(hasId('testEdge').inV().hasId('dddd')).
  fold().
  coalesce(unfold().property('testedder', 1111),
           addE('friend').from(V('pppp')).to(V('dddd')))

The above Gremlin assumes that you know "pppp" vertex exists. If you do not then you might try (as suggested by Daniel Kuppitz):

g.V('pppp').
  not_(outE('friend').hasId('testEdge').
       filter(inV().hasId('dddd')).
         property('testedder', 1111)).as('p').
  V('dddd').
    addE('friend').from('p')