How can I merge two mongo collections?

Question

I am banging my head against the wall on this...

SEE UPDATE 1 (below) !

I am merging two collections together... I looked at this example ( and ~several~ other examples here on SO ... )

https://docs.mongodb.com/manual/reference/operator/aggregation/lookup/#lookup-single-equality

I think I am really close, but my expected results are not the same as what I would expect out of the example.

Here is the schema for 'Event'

const EventSchema = new Schema({
    name: {type: String, required: true},
})

Here is some 'Event' data

[
    {
        "_id": "5e8e4fcf781d96df5c1f5358",
        "name": "358 Event"
    },
    {
        "_id": "5e8e55c5a0f5fc1431453b5f",
        "name": "b5f Event"
    }
]

Here is 'MyEvent' schema:

const MyEventSchema = new Schema({

    userId: {type: Schema.Types.ObjectId, required: true},
    eventId: {type: Schema.Types.ObjectId, required: true},
})

Here is some 'MyEvent' data

[
    {
        "_id": "5e8f4ed2ddab5e3d04ff30b3",
        "userId": "5e6c2dddad72870c84f8476b",
        "eventId": "5e8e4fcf781d96df5c1f5358",
    }
]

Here is my code ( the code is wrapped in a promise so it returns resolve and reject with data )

   var agg = [
       {
         $lookup:
           {
             from: "MyEvent",
             localField: "_id",
             foreignField: "eventId",
             as: "userIds"
           }
       }
    ];

   Event.aggregate(agg)
   .then( events => {
       return resolve(events);
   })
   .catch(err => {
       return reject(null);
   })

Here are my results,

[
    {
        "_id": "5e8e4fcf781d96df5c1f5358",
        "name": "358 Event",
        "__v": 0,
        "UserIds": []
    },
    {
        "_id": "5e8e55c5a0f5fc1431453b5f",
        "name": "b5f Event",
        "__v": 0,
        "UserIds": []
    }
]

I expect to see UserIds filled in for event '358 Event', like this What am I missing ???

[
    {
        "_id": "5e8e4fcf781d96df5c1f5358",
        "name": "358 Event",
        "__v": 0,
        "UserIds": [
          {"userId": "5e6c2dddad72870c84f8476b"}
         ]
    },
    {
        "_id": "5e8e55c5a0f5fc1431453b5f",
        "name": "b5f Event",
        "__v": 0,
        "UserIds": []
    }
]

UPDATE 1

I found a mongo playground and what I have works there, but it doesn't work in my code ??

https://mongoplayground.net/p/fy-GP_yx5j7

In case the link breaks, here is configuration: * select 'bson multiple collections'

db={
  "collection": [
    {
      "_id": "5e8e4fcf781d96df5c1f5358",
      "name": "358 Event"
    },
    {
      "_id": "5e8e55c5a0f5fc1431453b5f",
      "name": "b5f Event"
    }
  ],
  "other": [
    {
      "_id": "5e8f4ed2ddab5e3d04ff30b3",
      "userId": "5e6c2dddad72870c84f8476b",
      "eventId": "5e8e4fcf781d96df5c1f5358",

    }
  ]
}

Here is Query:

db.collection.aggregate([
  {
    $lookup: {
      from: "other",
      localField: "_id",
      foreignField: "eventId",
      as: "userIds"
    }
  }
])

Here is the result:

[
  {
    "_id": "5e8e4fcf781d96df5c1f5358",
    "name": "358 Event",
    "userIds": [
      {
        "_id": "5e8f4ed2ddab5e3d04ff30b3",
        "eventId": "5e8e4fcf781d96df5c1f5358",
        "userId": "5e6c2dddad72870c84f8476b"
      }
    ]
  },
  {
    "_id": "5e8e55c5a0f5fc1431453b5f",
    "name": "b5f Event",
    "userIds": []
  }
]

any suggestions as to why this doesn't work in my code... but works in the playground?

UPDATE 2

I found this:

Need a workaround for lookup of a string to objectID foreignField

UPDATE 3

I have changed the schema to use ObjectId for ids now still doesn't work

And they are ObjectIds :

RESOLUTION:

So the real answer was a combination of Update 2 and Update 3 and using the right collection name in the lookup.

Update 2 is pretty much my very same question... just using different table names

Update 3 is the correct way to solve this issue.

Mohammed Yousry pointed out the collection name might be wrong... so I looked at my schema and I did have it wrong - changed the name to the right name (along with ObjectId types) and it worked !

Answer 1

It seems there's a typo in from property in $lookup, MyEvent maybe not the collection name

db.collection.aggregate([
  {
    $lookup: {
      from: "MyEvent", // here is the issue I think, check the collection name and make sure that it matches the one you write here
      localField: "_id",
      foreignField: "eventId",
      as: "userIds"
    }
  }
])

in mongo playground you attached in the question, if you change the 'other' in the $lookup to anything else, or make a typo in it .. like others instead of other, you will face the same issue

so check that there is no typo in the word MyEvent that you populate from