MongoDB, Mongoose find all where _id is equal to key of object in array

Question

Struggling with a query, I have a collection of users:

{
    "_id": "5c87bda25fdaasdf00171001e1",
    "name": "UserTwo Name",
    "email": "email2@example.com"
},{
    "_id": "5c87bda25fda8b00171001e1",
    "name": "User Name",
    "email": "email@example.com"
}

I also have an array of objects containing a _userId key. I need to get all users where the _id of the user equals the value of _userId in my array.

[
    {_userId: "5c87bda25fda8b00171001e1"},
    {_userId: "5c87bda25fdaasdf00171001e1"},
]

What I have so far is:

User.find(
        {
            _id: { $in: req.body.users }
        }
    )

Which should be fine if req.body.users was just an array of ids, not array of objects.

Any help would be greatly appreciated. Thanks,

Answer 1

You should modify your query like this:

 User.find({
            _id: { $in: req.body.users.map(user => ObjectId(user._userId) }
 });

You need to convert _userId string to ObjectId. And that will be done with map function.

Answer 2

You have to turn the array

[
    {_userId: "5c87bda25fda8b00171001e1"},
    {_userId: "5c87bda25fdaasdf00171001e1"},
]

into

[
    "5c87bda25fda8b00171001e1",
    "5c87bda25fdaasdf00171001e1",
]

So how can we do that ?

---

Example 1 : using Array.map which iterate through the specified array, call a function and then create a new array with values being the ones returned by the function it called.

 console.log([{ _userId: "5c87bda25fda8b00171001e1", }, { _userId: "5c87bda25fdaasdf00171001e1", }, ].map(x => x._userId)); 

---

Example 2 : Using a well known for loop and create the array ourselves

 const arr = [{ _userId: "5c87bda25fda8b00171001e1" }, { _userId: "5c87bda25fdasdf0017101e1" }, ]; const newArr = []; for (let i = 0; i < arr.length; i += 1) { newArr.push(arr[i]._userId); } console.log(newArr); 

---

PS : As @Ivan Vasiljevic proposed, when building the array of id, you can turn the string into ObjectID objects. It's not mandatory to do it using mongoose.find ; it is tho using mongoose.aggregate .