Removing null objects from json in mongodb

Question

I am using php and MongoDB, the opensource library that i am using makes an auto-generated code to make calls to mongodb. it is sort of an ODM layer.

Now when i am using embedded documents, in the parent document, i have to update the set of embedded documents, the Library makes a call using $unset, which as per Documentation sets the value at index to null and does not remove it.

thus now when i want to replace the embedded document set, i have to first remove($unset) the existing set, and then add new leaving me with the result like this:

Original:
{
    "parentDocument": {
        "parent_id": "1",
        "embeddedDocument": {
            "0": {
                "childValue": "0"
            },
            "1": {
                "childValue": "1"
            },
            "2": {
                "childValue": "2"
            }

        }
    }
}

Updated:

{
    "parentDocument": {
        "parent_id": "1",
        "embeddedDocument": {
            "0": null,
            "1": null,
            "2": {
                "childValue": "2"
            }

        }
    }
}

How can i clean this data from the db..?? I have tried many forums, didnt find any valid solution for the same. I need to clean this complete data. Thanks

Benjamin M · Accepted Answer · 2015-04-07 19:12:44Z

1

You should have used $pull instead of $unset.

$unset will set elements to null inside arrays
$pull will remove elements from arrays

You can remove null elements using $pull:{'a.b':null}

answered Apr 7, 2015 at 19:12

Benjamin M

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5 Comments

Th3Nic3Guy Over a year ago

I tried $pull, works on array elements only. which means, that i remove the complete set first and then add back the elements that i wish to keep. Not a valid option...Or am i missing something

Th3Nic3Guy Over a year ago

I will check the last line again...might be useful...will revert soon

Benjamin M Over a year ago

Yes, indeed it just works with arrays. Is there a reason why you didn't use an array? "0", "1" and "2" as keys does look like an array could make sense. ... for normal docs and subdocs $unset should remove the key.

Th3Nic3Guy Over a year ago

The embedded set is an array, that is correct, but the elements of the array have to be removed individually. that means value[0],value[1],value[2] to be removed and to be left with value[3] at the 0th index. but right now it remains at 3rd index and previous values are null

Benjamin M Over a year ago

If it is an array, it should be written [{"childValue": "0"},{"childValue": "1"},{"childValue": "2"}]. Now the question is how you'd like to identify the elements to remove. If you need to remove it by array index, the only way is to use $unset and then $pull all null elements ( stackoverflow.com/questions/4588303/… ). If you can remove the elements by some known value, you can just use $pull ( stackoverflow.com/questions/16959099/… )

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