MongoDb: Query not pulling item from array - why?

Question

The intention of the query below is to pull items from the locs array, where x=2 and y=9. However items with these values remain in the array after this query.

 db.myCollection.update(
       { }, //All records
       { $pull: { 'locs' : { $elemMatch : {'x' : 2 , 'y' : 9 } } } }
 )

Could anyone tell me why it's not working?

Edit: Example document:

{
  "_id" : ObjectId("55555555555"),
  "locs" : [{
      "x" : 2,
      "y" : 9
    }],
  "v" : 99
}

Can you please add an example document as well?

Derick
– Derick

2013-07-25 10:53:01 +00:00
Commented Jul 25, 2013 at 10:53 — Derick
– Derick, Commented Jul 25, 2013 at 10:53

Derick · Accepted Answer · 2013-07-25 10:56:47Z

7

In general, $pull does not work like this. It removes "a value from an array" (http://docs.mongodb.org/manual/reference/operator/pull/). You can not use $elemMatch here, but neither do you have to.

So if you have a document that looks like this:

{
    'title': 'example',
    'locs': [
        { x: 2, y: 12 },
        { x: 7, y: 9 },
        { x: 2, y: 9 },
    ]
}

The follow should remove your x:2 / y:9 value pair:

db.so.update(
    {},
    { $pull: { 'locs' : { 'x' : 2 , 'y' : 9 } } }
);

Which then has:

{
    'title': 'example',
    'locs': [
        { x: 2, y: 12 },
        { x: 7, y: 9 },
    ]
}

answered Jul 25, 2013 at 10:56

Derick

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2 Comments

UpTheCreek Over a year ago

Ah I see, thanks for the clarification Derick. I think I need to read up on a few points including the purpose of elemMatch.

Derick Over a year ago

I think you understand $elemMatch just fine, it's just that $pull just wants values, and not expressions/queries.

Nicolas Rinaudo · Accepted Answer · 2013-07-25 10:59:58Z

2

I believe you need not use $elemMatch. Try with the following, it should work:

db.myCollection.update({}, {$pull: {'locs' : {'x' : 2 , 'y' : 9}}})

EDIT: or @Derick's answer, which is rather better than mine.

answered Jul 25, 2013 at 10:59

Nicolas Rinaudo

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