MongoDB: find if ID exist in array of objects

Question

I was wondering, is there a way in MongoDB to get all the documents from one collection excluding those that exists in another, without using aggregation.

Let me explain myself more;

I have a collection of "Users", and another collection where there is an array of objects. Something like this:

User

  }
    "_id": "61e6bbe49d7efc57f895ab50",
    "name": "User 1"
  },
  {
    "_id": "61e6b9239d7efc57f895ab02",
    "name": "User 2"
  },
  {
    "_id": "61cae6176d0d9a36efd8f190",
    "name": "User 3"
  },
  {
    "_id": "61cade886d0d9a36efd8f11a",
    "name": "User 4"
  },

The other collection looks like this:

{
    users: [
        {
           user: {
              "_id": "61e6b9239d7efc57f895ab02",
              "name": "User 2",
           },
           ...

        },
        {
           user: {
              "_id": "61cae6176d0d9a36efd8f190",
              "name": "User 3",
           },
           ...
        },
    ],
},

I would like to get all the users in "array 1" excluding those in "array 2". So the result should be:

    [
        {
           "_id": "61e6b9239d7efc57f895ab02",
           "name": "User 1"
        },
        {
           "_id": "61cae6176d0d9a36efd8f190",
           "name": "User 4"
        },
    ],

So, is there a way to do that without the need to do aggregation.

You can fire two queries if you don't want aggregation

Gibbs
– Gibbs

2022-02-02 17:03:43 +00:00
Commented Feb 2, 2022 at 17:03 — Gibbs
– Gibbs, Commented Feb 2, 2022 at 17:03

Zohar · Accepted Answer · 2022-02-03 04:01:19Z

1

you can use the $and like this:

const found = await User.find({
    $and: [
      { _id: { $in: array1 } },
      { _id: { $nin: array2 } }
    ]
  });

answered Feb 3, 2022 at 4:01

Zohar

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1 Comment

Aymen Fezai Over a year ago

Thanks for your help ... your answer is kinda what I already wrote, but when I used to add a new element to the array I didn't initialize the "_id" so mongo gives a default _id to each new element ... all I had to do is modify that custom behavior and initialize the _id with the user's _id.

Aymen Fezai · Accepted Answer · 2022-02-03 08:57:16Z

1

For anyone looking for a solution, if you want to add new element to an array of objects, where you, somehow, forgot to initialize the _id field, remember you have to, because mongo adds that field automatically (to use it for filtering/indexation).

All you have to do, is to write something like this;

const data = Data.create({
    _id: mongoose.Types.ObjectId.createFromHexString(uid),
    users: [{ user: userId, initiator: true, _id: userId}],
})

and for the searching, you write what you always write;

const found = await User.find({ _id: { $nin: data.users } });

answered Feb 3, 2022 at 8:57

Aymen Fezai

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