Move an array element from one array position to another (mongo)

Question

I have a list like this:

{ 
    arr: [
        { _id: 'a', val: 1 }, 
        { _id: 'b', val: 4 }, 
        { _id: 'd', val: 0 }, 
        { _id: 'c', val: 8 }
    ] 
}

And I need to put the element {_id: c} before the element {_id: d}. How can I do it with mongo? Of course, I can download the doc to my server, prepare it and upload the one to the mongo server. But I think, it's not a good way.

P.S: sorry, I need to move the element (push it to a new position and remove the one from old position)

I think a easier way of doing that would be to make another id that you can more easily access, and sort by that id when you need to use the document. — DrakaSAN
– DrakaSAN, Commented Jun 17, 2016 at 12:36
Do you want to swap elements or to sort them? If you need to sort them there is a $sort option you may be able to use. — Soren
– Soren, Commented Jun 17, 2016 at 13:06

Community · Accepted Answer · 2017-05-23 11:52:50Z

1

You can use the keyword $position to specify where in the array you want to insert the value. Use it when $pushing an element on and you'll be able to insert that element into a specific place in the array.

db.collection.update(
    {parameter: doc},
    {$push: {
        arr: {
            $each: [{_id: "c", val: 8}],
            $position: 2
        }
    })

This will push that object into position 2 of your array. If you're looking to manipulate the array by removing objects at specific points, you can refer to the answer I linked in the comments.

edited May 23, 2017 at 11:52

CommunityBot

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answered Jun 17, 2016 at 12:49

Gibryon Bhojraj

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5 Comments

Soren Over a year ago

NIce, how does he remove the element in position 3? Maybe a working example would be good.

Gibryon Bhojraj Over a year ago

There are more ways to do that, using $pull. I don't know if you can specify a $position field within $pull, but there are other answers for that: stackoverflow.com/questions/4969768/…

Shantanu Madane Over a year ago

And what about the document which is at position 2

Gibryon Bhojraj Over a year ago

$push won't remove the document at position 3, and OP didn't ask to remove it, so this code preserves the array and just inserts. But @Soren asked about removing the element at a certain position, so I included a link to do that with $pull

DecPK Over a year ago

The author of this question specifically asked to shift the element from one position to another, not to push a new element. This is half right answer.

Shantanu Madane · Accepted Answer · 2016-06-17 12:51:11Z

0

Try This:

db.myCollection.find().forEach(function(doc){
        db.myCollection.update({_id: doc._id},
          {$set: {"arr.2": doc.arr[3], "arr.3": doc.arr[2]}})
    })
OR
You can Pull and then push those documents

answered Jun 17, 2016 at 12:51

Shantanu Madane

6155 silver badges15 bronze badges

4 Comments

Alex Ivashkin Over a year ago

>> Of course, I can download the doc to my server, prepare it and upload the one to the mongo server. But I think, it's not a good way.

Soren Over a year ago

Does the callback not copy this to the application rather than just work on the mongodb server?

Alex Ivashkin Over a year ago

@Soren yeap, but if I will work on the mongodb server I will block the main thread.

Soren Over a year ago

Not sure that is relevant, since the OP specifically wanted something that didn't copy the data across.

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