mongodb aggregation lookup using ObjectId - mongodb

Question

I am struggling with what seems to a basic lookup in mongodb. I can't figure out what is going wrong after trying multiple combinations. I see that there are many questions on SO addressing it, but nothing works after trying all the answers.
Here's my users collection
Here's my items collection
Each item has a single owner which maps to the user's _id in users collection.
For my query, I am trying to fetch items with their corresponding users. This is the query
db.items.aggregate([
{
"$lookup" : {
localField : "owner",
from : "users",
foreignField : "_id",
as : "users"
}
}
])
It returns this -
I have tried different variations - such as
db.items.aggregate([
{
"$lookup" : {
localField : "owner.str",
from : "users",
foreignField : "_id.str",
as : "users"
}
}
])
which results in the array of users getting populated incorrectly (with all users!).
What am I doing wrong?
EDIT
here's the items collection
{
"_id": {
"$oid": "5b268395c176db1548bd92c2"
},
"title": "Item #1",
"description": "Item 1 Description",
"categories": [
{
"$ref": "categories",
"$id": {
"$oid": "5b268248c176db1548bd92af"
}
}
],
"status": "borrowed",
"image": "http://cdn.site.com/testimage.png",
"borrower": "5b2684a0c176db1548bd92d5",
"owner": {
"$ref": "users",
"$id": {
"$oid": "5aecc8012d3d947ba130800d"
}
}
}
{
"_id": {
"$oid": "5b2c2c4d3c70af2da07d1266"
},
"title": "Item 2",
"description": "Item 2 Description",
"categories": [
{
"$ref": "categories",
"$id": {
"$oid": "5b268248c176db1548bd92af"
}
}
],
"status": "Available",
"image": "http://cdn.site.com/testimage1.png",
"borrower": null,
"owner": {
"$ref": "users",
"$id": {
"$oid": "5b2684a0c176db1548bd92d5"
}
}
}
Here's the users collection :
{
"_id": {
"$oid": "5aecc8012d3d947ba130800d"
},
"email": "n.y#gmail.com",
"registeredOn": "20/10/2018 12:12:29 AM",
"avatarURL": "http://gravtar.com/12334",
"friends": []
}
{
"_id": {
"$oid": "5b2684a0c176db1548bd92d5"
},
"email": "j.v#gmail.com",
"registeredOn": "20/10/2018 12:12:29 AM",
"avatarURL": "http://gravtar.com/12334",
"friends": [],
"wishlist": [
{
"$ref": "items",
"$id": {
"$oid": "5b2831543c70af2da07d11da"
}
}
]
}

Answer 1

The localField and foreignField should be of the same data type. Your are DBRef and ObjectId respectively.

Answer 2

If you want to populate a items record with the corresponding user, I think you can use the populate option in mongoose find API
Sample code:
this.read = function(query, populate, onSuccess, onFailure, forcePull, sortQuery, ttl, limitValue, selectQuery) {
var scope = this;
logger.info(this.TAG, "Read operation execution started on ORM Object: "
+ modelName + " with query: " + JSON.stringify(query));
if (query) {
var queryObject = this._model.find(query);
if (populate) {
queryObject = queryObject.populate(populate);
}
if (selectQuery) {
queryObject = queryObject.select(selectQuery);
}
if (sortQuery) {
queryObject = queryObject.sort(sortQuery);
}
if (limitValue) {
queryObject = queryObject.limit(limitValue);
}
queryObject.exec(function(error, records) {
if (error) {
logger.error(scope.TAG, "Error while ORM read: " + JSON.stringify(error));
onFailure(error);
return;
}
logger.info(scope.TAG, "Record read successfully: " + JSON.stringify(records));
onSuccess(records);
});
}
}
You can pass populate as an array of strings like ['owner'] in your case