How do I remove just one Element from MongoDB Array that equals a value?

Question

I have an Array of ints in mongo db. Some of the values come up multiple times like this:

{binder : [4,7,9,7]}

I use pull on the collection like this

{ $pull: { binder: 7} }

It will remove all the 7 and I end up with:

{binder : [4,9]}

However I want to just remove one of the sevens to get something like this:

{binder : [4,7,9]}

How would I go about this. The indices of the numbers are not known and duplicates are not always on last/first spots.

After trying and searching for a long time I found a way with $indexOfArray which is not supported where I need to use it.

Kevin Smith · Accepted Answer · 2016-12-10 21:23:30Z

1

There is currently no way to only remove one item from an array, as said in the documention "The $pull operator removes from an existing array all instances of a value or values that match a specified condition."

You can however use this work around:

Find and unset one item from the array:

> db.arrays.save({ s : [ 1, 2, 3, 4, 5, 5, 6, 7, 8, 9, 10 ] })
WriteResult({ "nInserted" : 1 })

> db.arrays.update({ "s" : 5 }, { $unset : { "s.$" : true } })
WriteResult({ "nMatched" : 1, "nUpserted" : 0, "nModified" : 1 })

This will give us the following

> db.arrays.find()
{ "_id" : ObjectId("584c707f1c86f44b7300b223"), "s" : [ 1, 2, 3, 4, null, 5, 6, 7, 8, 9, 10 ] }

Then we can just pull the nulls

> db.arrays.update({ }, { $pull: { "s" : null } })
WriteResult({ "nMatched" : 1, "nUpserted" : 0, "nModified" : 1 })

Our 5 that we remove now will be gone:

> db.arrays.find()
{ "_id" : ObjectId("584c707f1c86f44b7300b223"), "s" : [ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 ] }

It's a bit of a work around but it's pretty safe with multiple atomic operations (as long as you don't use null as a valid value)

answered Dec 10, 2016 at 21:23

Kevin Smith

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2 Comments

1knueller Over a year ago

This worked great. Thanks. Just one more question. Is using an array like that just not good? Is there a better way to simply store numbers - or did they just not implement this as function yet because there are workarounds?

Kevin Smith Over a year ago

It totally depends on your use case, if you had a more complex object within the array you could just append a unique _id so that you could get a handle to the item you wish to remove. Just had a quick search and there is an issue in Jira for this - jira.mongodb.org/browse/SERVER-1014

rpstw · Accepted Answer · 2021-08-13 09:21:31Z

0

Use $pullAll in newer version of mongo.

https://docs.mongodb.com/manual/reference/operator/update/pullAll/

The top answer is outdated.

answered Aug 13, 2021 at 9:21

rpstw

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