11

My main concern is about in mongodb documentation they use $elemMatch command for find and pull the element from array, but this is not work when i use. My document structure is

{
"_id" : ObjectId("55dea445c3ad8cac03a7ed8e"),
"email" : "james@user.com",
"groups" : [ 
    {
        "_id" : ObjectId("55dea4a4c3ad8c8005a7ed8f"),
        "name" : "All Group"
    }
]}

I want to remove groups from document using mongodb query. My Query is : 

db.users.update({"_id": ObjectId('55dea445c3ad8cac03a7ed8e')}, 
{$pull: {"groups": {$elemMatch: {"_id": ObjectId('55dea4a4c3ad8c8005a7ed8f')}}}})

After executing the query, the user document not updated and groups value still there. I am following mongodb documentation as: http://docs.mongodb.org/v2.6/reference/operator/update/pull/

Harmeet Singh Taara
  • 6,483
  • 20
  • 73
  • 126
  • possible duplicate of [How to remove array element in mongodb?](http://stackoverflow.com/questions/16959099/how-to-remove-array-element-in-mongodb) – Neo-coder Aug 27 '15 at 06:53
  • 1
    possible duplicate of [MongoDB, remove object from array](http://stackoverflow.com/questions/15641492/mongodb-remove-object-from-array) – Vishwas Aug 27 '15 at 06:55

1 Answers1

18

You do not need $elemMatch with $pull. It's arguments are basically a "query" on the array itself:

db.users.update(
    {"_id": ObjectId('55dea445c3ad8cac03a7ed8e')}, 
    { "$pull": { "groups": {"_id": ObjectId('55dea4a4c3ad8c8005a7ed8f')}}}
)

So $pull works much in way an $elemMatch works within a query, where it's own arguments are treated as a query in the subdocument elements of the array.

It is looking at the individual array elements already, unlike a "query" portion in the document which sees the whole array.

Blakes Seven
  • 49,422
  • 14
  • 129
  • 135
  • Thanks @Blakes Seven this is work for me. But i am confuse about mongodb documentation, because in documentation they use `$elemMatch` query. – Harmeet Singh Taara Aug 27 '15 at 06:58
  • @HarmeetSinghTaara No they do not. I know the piece of documentation you are referring to but if you read carefully it actually says: *" In fact, the following operation will not pull any element from the original collection."*. So it is in fact meant to be an example of what **not** to do, as people make the common mistake of *assuming* `$elemMatch` needs to be applied. It does not. Read carefully. – Blakes Seven Aug 27 '15 at 07:03
  • Thanks @Blakes Seven, i have query more, how we remove doucment from array of array like mention in documentation as below: `{ _id: 1, results: [ { item: "A", score: 5, answers: [ { q: 1, a: 4 }, { q: 2, a: 6 } ] }, { item: "B", score: 8, answers: [ { q: 1, a: 8 }, { q: 2, a: 9 } ] } ] }`. Query is : `db.survey.update( { }, { $pull: { results: { $elemMatch: { score: 8 , item: "B" } } } }, { multi: true } )` – Harmeet Singh Taara Aug 27 '15 at 07:10
  • 2
    @HarmeetSinghTaara Well you can always [ask another question](http://stackoverflow.com/questions/ask), which is generally best rather than asking in comments. But the answer is faily well described in the documentation. – Blakes Seven Aug 27 '15 at 07:16