Mongo how to $lookup with DBRef

Question

I have a trouble(/(ㄒoㄒ)/~~). Suppose that collection A is

{ 
    \"_id\" : ObjectId(\"582abcd85d2dfa67f44127e1\"), 
    \"bid\" : [
        DBRef(\"B\",         


        

Answer 1

As of mongoDB 3.4, this is not possible. You can't use DBRef in the aggregation pipeline, except in the $match stage.

I strongly recommend you to get rid of DBRef and switch to manual references. However, if you really need to keep DBRef, here is an (ugly) solution:

first, create a new collection named "C", where DBRefs are replaced by their Ids using mapReduce:

db.A.mapReduce(
    function() {
        var key = this._id; 
        var value = [];  
        for ( var index = 0; index < this.bid.length; index++){
           value.push(this.bid[index].$id); 
        }
        emit(key, value); 
    },
    function(key,values) {
        return  values;
    },
    {
        "query": {},
        "out": "C" 
    }
)

then, run your aggregation query on the new "C" collection:

db.C.aggregate([
   {
      $unwind:"$value"
   },
   {
      $lookup:{
         from:"B",
         localField:"value",
         foreignField:"_id",
         as:"bs"
      }
   }
]);

output:

    {
       "_id":ObjectId("582abcd85d2dfa67f44127e1"),
       "value":ObjectId("582abcd85d2dfa67f44127e0"),
       "bs":[
          {
             "_id":ObjectId("582abcd85d2dfa67f44127e0"),
             "status":1,
             "seq":0
          }
       ]
    }{
       "_id":ObjectId("582abcd85d2dfa67f44127e1"),
       "value":ObjectId("582abcd85d2dfa67f44127e1"),
       "bs":[
          {
             "_id":ObjectId("582abcd85d2dfa67f44127e1"),
             "status":1,
             "seq":0
          }
       ]
    }