How do I remove an element in an array based on content?

Question

I am working with MongoDB and Perl. Here is my data structure:

{
    "_id" : ObjectId("501976f8005c8b541d000000"),
    "err_id" : "err",
    "solution" : [
        {
            "attachment" : "attach",
            "macr" : "macrs",
            "yammer" : "yam",
            "resolution" : "l",
            "salesforce" : "salesforce",
            "username" : "bob"
        },
        {
            "attachment" : "attach",
            "macr" : "macrs",
            "yammer" : "yam",
            "resolution" : "losssss",
            "salesforce" : "salesforce",
            "username" : "bob"
        }
    ]
}

As you can see, I have an array with objects inside. I have created this using the Perl MongoDB library.

I am familiar with some syntax for manipulating arrays in the Perl MongoDB lib. For example, I use this to find entries with a username the same as $username.

$users->find({"solution.username" => $username});

I thought removing an element would be as simple:

$users->remove({"solution.username" => $username});

But alas, it is not so. I have tried this and using pull, but to no avail! I've had a hard time finding this. Does anybody know the syntax to remove an array element based on the contents of one of its fields?

Just like find() returns the whole document, remove() removes the whole document.. — Aafreen Sheikh
– Aafreen Sheikh, Commented Aug 1, 2012 at 18:55
Right on - as it were I actually want to remove the whole document! But it seems that it isn't removing anything at all. Is my example syntax the correct syntax? — PinkElephantsOnParade
– PinkElephantsOnParade, Commented Aug 1, 2012 at 18:56
I'm unfamiliar with Perl syntax, but I checked the documentation. It looks ok..If your find() DOES return some documents, then your remove() SHOULD ideally remove all these documents, because your query is exactly the same, as you mention in the example. — Aafreen Sheikh
– Aafreen Sheikh, Commented Aug 1, 2012 at 19:03
Related questions:stackoverflow.com/questions/4969768/…, stackoverflow.com/questions/7227890/… — Aafreen Sheikh
– Aafreen Sheikh, Commented Aug 1, 2012 at 19:19
Have you tried checking the error messages? You should call last_error() after your run to make sure there wasn't a problem or run $users->remove({"solution.username" => $username},{safe => 1}); which will croak on error. — zostay
– zostay, Commented Aug 1, 2012 at 19:53

Stennie · Accepted Answer · 2012-08-02 07:07:25Z

1

The MongoDB::Collection remove() method will remove documents matched by your query .. so definitely not what you are looking for.

To delete specific fields you should use $unset.

Your solution.usernames are actually in an array, so you would have to include an array index for the fields to delete, eg:

$users->update({"_id" => '123'}, {
   '$unset' => {
       'solution.0.username' => 1,
       'solution.1.username' => 1
   }
});

I'm not aware of a shorter syntax to unset all fields matching username within the solution array, but you can add multiple solution.#.username fields to the $unset command.

My example above deletes the first two username entries from the array. If the matching document(s) had more than two username entries, each time you ran this update you would delete up to two more entries (if they exist).

answered Aug 2, 2012 at 7:07

Stennie

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