How to join 2 collections with array of string object ID mongodb

Question

I have a collection called store

{
  "_id": {
    "$oid": "64f885a4758688440f24d070"
  },
  "name": "Test Location",
  "Tags": [
    "66249849f94df523bfb6324d",
    "6624984ff94df523bfb6324e"
  ],
  "updatedAt": {
    "$date": "2024-07-23T22:30:38.454Z"
  }
}

And I have a Tag collection

{
  "_id": {
    "$oid": "66249849f94df523bfb6324d"
  },
  "name": "Night",
}

{
  "_id": {
    "$oid": "6624984ff94df523bfb6324e"
  },
  "name": "Bar",
}

I would like to join 2 collections with tag ids to something like

{
  "_id": {
    "$oid": "64f885a4758688440f24d070"
  },
  "name": "Test Location",
  "Tags": [
    "66249849f94df523bfb6324d",
    "6624984ff94df523bfb6324e"
  ],
  "TagResult": [
     {"name": "Night"},
     {"name": "Bar"},
  ] 
}

Any help is appreciated.

Answer 1

You probably had a failed attempt with $lookup since you have a mismatch in the datatypes of the 2 collections. (i.e. string vs ObjectId). You need to convert them to same datatype to perform the $lookup.

db.store.aggregate([
  {
    "$set": {
      "Tags": {
        "$map": {
          "input": "$Tags",
          "as": "t",
          "in": {
            "$toObjectId": "$$t"
          }
        }
      }
    }
  },
  {
    "$lookup": {
      "from": "tag",
      "localField": "Tags",
      "foreignField": "_id",
      "as": "TagResult"
    }
  }
])

Mongo Playground

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Side note: if possible, refactor the schema to store the ids in the same datatype(i.e. either both ObjectIds or both strings) so you don't have to do the type conversion at run time.