group with lookup's foreign field in mongodb

You can try below aggregate function to achieve the result...

If you have mongodb version 3.6

db.promotion.aggregate([  // stage 1  { "$lookup": {    "from": Promotions.collection.name,    "let": { "tags": "$tags" },    "pipeline": [       { "$match": { "$expr": { "$eq": [ "$_id", "$$tags" ] } } }     ],     "as": "tags"  }},  // stage 2  { "$addFields": {     "tags": { "$arrayElemAt": [ "$tags", 0 ] }  }},  // stage 3  { "$group": {    "_id": "$tags._id",    "promotions": {        "$push": {            "fieldName1": "$fieldName1",            "fieldName2": "$fieldName2",        }    }  }} ])

If you have mongodb version prior to 3.6

db.promotion.aggregate([  { "$lookup": {    "from": Promotions.collection.name,    "localField": "tags",    "foreignField": "_id"     "as": "tags"  }},  { "$unwind": "tags" },  { "$group": {    "_id": "$tags._id",    "promotions": {        "$push": {            "fieldName1": "$fieldName1",            "fieldName2": "$fieldName2",        }    }  }} ])

Both will give similar output

{    "tag": {     "_id": "999",     "value": "Lorem Ipsum"   },   "promotions": [{      "_id": "0001",      "value": "Value of promotion Nº1"   },   {      "_id": "0002",      "value": "Value of promotion Nº2"   }   ]}}