从文档中的数组中拉出

Question

I am trying to $pull from an array within a mongodb document.

The document has the structure:

{
    "_id" : ObjectId("54ee62ef688b41ff072b934b"), 
    "pictures" : [ 
        {
            "url" : "...", 
            "_id" : ObjectId("54ee6303688b41ff072b934d") 
        }, 
        { 
            "url" : "...", 
            "_id" : ObjectId("54ee6304688b41ff072b934e")
        }, 
        { 
            "url" : "",
            "_id" : ObjectId("54ee6304688b41ff072b934f") 
        } 
    ]
}

I tried the update object

var update = { $pull: { pictures: {$elemMatch: {_id:req.params.picid } } } }

db.activity.update({_id: new ObjectId(req.params.id)}, update)

which returns writeresult: 1, but the picture is never removed.

ps I am using node, hense the req.params.picid

Answer 1

The $pull operator acts as a query document in itself and is also considered against every element of the array so $elemMatch is not needed:

var update = { 
   "$pull": { "pictures": { "_id": new ObjectId(req.params.picid) } }
};

You also need to cast your "string" from request params to an ObjectId .