MongoDB: Remove array elements from a document

Question

I'm new to Mongo and thus trying to implement a use-case wherein I need to build a query to remove data from an array from inside a collection of documents.

Here is one of my documents:

{
"id" : 2,
"owners" : ["aa", "bb"]
}

Now I need to build a query to find all document records based on the owners and if present, remove them from the owners array.

For example, I sent aa in the query, then I need to remove that from all the documents that contains that owner.

I found somewhere this logic:

$db.coll.update({cond to identify document},{$pull:{'owners':{'aa':<>}}})

But don't understand how does this remove the data. Thanks in advance!

PaulShovan · Accepted Answer · 2016-09-19 12:31:46Z

6

The following query updates a collection by removing the matched owners from owners array. The update query contains three blocks,

db.coll.update(
        { },
        { $pull: { owners: "aa" } },
        { multi: true }
    )

first block {}: it specify the query to find the documents, as we are considering all documents, we kept it blank

second block { $pull: { owners: "aa" } }: This one is removing matching elements from owners array from all documents.

third block { multi: true }: this block specifies that, we are considering update for all matching document, { multi: false } will update only the first matched document

To learn more you can follow this link

edited Sep 19, 2016 at 12:31

answered Sep 19, 2016 at 12:20

PaulShovan

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4 Comments

Abhishek Over a year ago

thanks for replying. Can you please explain what exactly is happening here?

chridam Over a year ago

A better approach would be to use a query condition in your update method in which the update operation only updates the documents matched by the query and use an index, rather than do a full collection scan and checks for each document in the collection then do a $pull which could be potentially slow on large datasets. Your solution can be further improved by including the query as db.coll.update( { "owners": "aa" }, { "$pull": { "owners": "aa" } }, { "multi": true } )

PaulShovan Over a year ago

@chridam yes, it is true

Abhishek Over a year ago

Thanks very much gypsyCoder and chridam.

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