mongodb query using _id object in lookup localField

Question

we are trying to match _id object to mId foreignfield which is not working. looking for mondodb docs. they do not have anything on this. Is it possible with mongodb query or not?

_id as object in document

   "_id" : ObjectId("56ab6663d69d2d1100c074db"),

mId as String in document

"mId" : "56ab6663d69d2d1100c074db",

query as below:

 collection.aggregate([
                {
                  $lookup:
                    {
                      from: "category",
                      localField: "_id",
                      foreignField: "mId",
                      as: "categories"
                    }
               }
            ])

UDPATE

as a summary, mongodb does not support type coercion in $lookup. so to need above working i have to have _id and mId as ObjectId type in documents itself to make it work.

Answer 1

From Mongodb 4.0 onwards, you can use $toString aggregation operator to convert ObjectId to string.

Jira issue : Allow $lookup between ObjectId (_id.str) and string

---

Now your query should be like below :

collection.aggregate([
  { 
    $addFields: { "_id": { "$toString": "$_id" } }
  },
  {
    $lookup: {
      from: "category",
      localField: "_id",
      foreignField: "mId",
      as: "categories"
    }
  }
])