MongoDB insert a document for every documents in another collection

Question

I have a collection (users) containing some documents like so :

{
    _id: ObjectId("56d45406be05db4022be51f9"),
    morecontent : ""
},
{
    _id: ObjectId("56d45406be05db3021be32e3"),
    morecontent : ""
}

I would like to create a new document for every entry in the user collection. The documents would be created from a notification object like this :

{
    type: 'alert',
    msg: 'This is important'
}

The collection notifications should look something like this :

{
    _id: ObjectId("56d45406be05db3021bf20a1"),
    someoldcontent: "This was here before the request"
},
{
    _id : ObjectId("56d45406be05db4022be20b1"),
    user: ObjectId("56d45406be05db4022be51f9"),
    type: 'alert',
    msg: 'This is important'
},
{
    _id : ObjectId("56d45406be05db3021be32e3"),
    user: ObjectId("56d45406be05db3021be32e3"),
    type: 'alert',
    msg: 'This is important'
}

Is there any way to do this in a mongodb request?

what will you use as db-client? (node, java, c#) or you want just to have a query to perform that? — profesor79, Feb 29 '16 at 16:47
I am not sure that I got you question right 1. user performs action 2. applications makes insert into notifications collection using user _id — profesor79, Feb 29 '16 at 17:12
@profesor79 I want to insert an object in the notification collection for every user document there is in the user collection. The notification will be the same for every users except it should have a different user_id. The notifications should be in the notifications collection. — Samuel Rondeau-Millaire, Feb 29 '16 at 17:19
let's chat here http://chat.stackoverflow.com/rooms/104903/mongo-queries-issues — profesor79, Feb 29 '16 at 17:58

score 2 · Accepted Answer · edited Mar 26 '21 at 18:52

Thanks to professor79 for helping me a lot with the question.

After lots of effort, the query was found. We used the aggregation framework in order to succeed.

The only 2 aggregations needed are $project and $out. The $out will automatically take care of the new _id in the notification document.

Given this collection named user :

{
    _id: ObjectId("56d45406be05db4022be51f9"),
    morecontent : ""
},
{
    _id: ObjectId("56d45406be05db3021be32e3"),
    morecontent : ""
}

We want to create a notification containing the same msg and type field for every document in the user collection. Each notifications will be in the notifications collection an will have as a reference its corresponding userId.

Here is the query to achieve such a result :

db.user.aggregate([
    {
        $project: {
            userId: '$_id',
            type: { $literal: 'danger' },
            msg: { $literal: 'This is a message' },
        },
    },
    { $out: 'notifications' },
])

The output in notifications collection:

{
    "_id" : ObjectId("56d6112e197b4ea11a87de1a"),
    "userId" : ObjectId("56d45406be05db4022be51f9"),
    "type" : "danger",
    "msg" : "This is a message"
},
{
    "_id" : ObjectId("56d6112e197b4ea11a87de1b"),
    "userId" : ObjectId("56d45406be05db3021be32e3"),
    "type" : "danger",
    "msg" : "This is a message"
}

profesor79 · Answer 2 · 2016-02-29T18:54:31.333

1

As we have some chat to clarify problem:

the steps are needed to perform this operation server side:

first step - match > get users id

project ids to new documents as required

out -> store output in notification collection

edited Feb 29 '16 at 18:54

answered Feb 29 '16 at 17:10

profesor79

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MongoDB insert a document for every documents in another collection

2 Answers2