Q:

How to form inner SubQuery in Gremlin Server (Titan 1.0)?

I'm using Following Query :
g.V(741440).outE('Notification').order().by('PostedDateLong', decr).range(0,1).as('notificationInfo').match(
    __.as('notificationInfo').inV().as('postInfo'),
).select('notificationInfo','postInfo')

it is giving following result :
{

"requestId": "9846447c-4217-4103-ac2e-de3536a3c62a",
"status": {
    "message": "",
    "code": ​200,
    "attributes": { }
},
"result": {
    "data": [
        {
            "notificationInfo": {
                "id": "c0zs-fw3k-347p-g2g0",
                "label": "Notification",
                "type": "edge",
                "inVLabel": "Comment",
                "outVLabel": "User",
                "inV": ​749664,
                "outV": ​741440,
                "properties": {
                    "ParentPostId": "823488",
                    "PostedDate": "2016-05-26T02:35:52.3889982Z",
                    "PostedDateLong": ​635998269523889982,
                    "Type": "CommentedOnPostNotification",
                    "NotificationInitiatedByVertexId": "1540312"
                }
            },
            "postInfo": {
                "id": ​749664,
                "label": "Comment",
                "type": "vertex",
                "properties": {
                    "PostImage": [
                        {
                            "id": "amto-g2g0-2wat",
                            "value": ""
                        }
                    ],
                    "PostedByUser": [
                        {
                            "id": "am18-g2g0-2txh",
                            "value": "orbitpage@gmail.com"
                        }
                    ],
                    "PostedTime": [
                        {
                            "id": "amfg-g2g0-2upx",
                            "value": "2016-05-26T02:35:39.1489483Z"
                        }
                    ],
                    "PostMessage": [
                        {
                            "id": "aln0-g2g0-2t51",
                            "value": "hi"
                        }
                    ]
                }
            }
        }
    ],
    "meta": { }
}

}

I want to get information of Vertex "NotificationInitiatedByVertexId" (Edge Property ) in the response as well.
For that i tried following query :
g.V(741440).outE('Notification').order().by('PostedDateLong', decr).range(0,2).as('notificationInfo').match(
  __.as('notificationInfo').inV().as('postInfo'),
  g.V(1540312).next().as('notificationByUser')
).select('notificationInfo','postInfo','notificationByUser')

Note : I tried directly with vertex Id in subquery as I wasn't aware how to dynamically get value from edge property in query itself.
It is giving error. I tried a lot but am not able to find any solution.

A:

I'm assuming that you are storing a Titan generated identifier in that edge property called NotificationInitiatedByVertexId. If so, please consider the following even though this first part doesn't really answer your question. I don't think you should store a vertex identifier on the edge. Your graph model should explicitly track the relationship of NotificationInitiatedBy with an edge and by storing the identifier of the vertex on the edge itself you are bypassing that. Also, if you ever have to migrate your data in some way, the ids won't be preserved (Titan will generate new ones) and trying to sort that out will be a mess. 
Even if that is not a Titan generated identifier and a logical one you created, I still think I would look to adjust your graph schema and promote that Notification to a vertex. Then your Gremlin traversals would flow more easily.
Now, assuming you don't change that, then I don't see a reason to not just issue two queries in the same request and then combine the results to one data structure. You just need to do a lookup with the vertex id which is going to be pretty fast and inexpensive:
edgeStuff = g.V(741440).outE('Notification').
              order().by('PostedDateLong', decr).range(0,1).as('notificationInfo').
              ...  // whatever logic you have
              select('notificationInfo','postInfo').next()
vertexStuff = g.V(edgeStuff.get('notificationInfo').value('NotificationInitiatedByVertexId')).next()
[notificationInitiatedBy: vertexStuff, notification: edgeStuff]

