Doctrine ODM: create $lookup on aggregated field with aggregation builder

Question

In a simplified data model, I have three types of documents: items, users and assignments. Users and items are stored in their own collections, while assignments are embedded in items. A sample item might look like this:

{
    "_id" : ObjectId("xxx"),
    "name" : "yyy",
    "assignments" : [ 
        {
            "assignmentDate" : ISODate("2018-01-11T10:05:20.125Z"),
            "user" : ObjectId("zzz"),
        },
        {
            "assignmentDate" : ISODate("2018-01-12T10:05:20.125Z"),
            "user" : ObjectId("iii"),
        }
    ]
}

I would like to query all items that are currently assigned to a given user. This aggregation pipeline does the job:

db.Item.aggregate([
    {
        $addFields: {
            currentAssignment: { $arrayElemAt: ['$assignments', -1] }
        }
    },
    {
        $lookup: {
            from: 'User',
            localField: 'currentAssignment.user',
            foreignField: '_id',
            as: 'currentUser'
        }
    },
    {
        $match: {
            'currentUser.name': { $eq: 'admin' }
        }
    }
]);

How can I build this with the Doctrine ODM Aggregation Builder? The Stage::lookup method accepts only a from parameter. If I use it on a computed field from the aggregation pipeline (currentAssignment in this case), it results in:

arguments to $lookup must be strings, localField: null is type null

Other solutions (if possible even without aggregation?) for retrieving the described dataset are also welcome.

Answer 1

The Lookup stage has more methods, one of which is localField which sets the localField in the aggregation stage.