MongoDb: Difference between $push/$addtoset

Question

I read the documentation in the MongoDb and I used a simple proves and I only look that: Push is sorting the array but addtoSet isn't it.

For me visually is the same, I don't know the difference.

Could anybody explain me the difference?

Another think if it could be in spanish or in a simple english, i'll aprecite it.

`$push` never sort an array. Both `$push` and `$addToSet` add new elements to the end on a target array. — Leonid Beschastny, Dec 02 '14 at 11:50

score 153 · Accepted Answer · edited Aug 04 '20 at 11:17

153

$addToSet do not add the item to the given field if it already contains it, on the other hand $push will add the given object to field whether it exists or not.

{_id: "docId", items: [1, 2]}
db.items.update({_id:"docId"}, {$addToSet:{items: 2}}); // This won't update the document as it already contains 2
db.items.update({_id:"docId"}, {$push: {items:2}}); // this will update the document. new document {_id: "docId", items:[1,2,2]}

edited Aug 04 '20 at 11:17

Kaspar Lee

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answered Dec 02 '14 at 11:47

cubbuk

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1

Thanks for your explanation now i'm doing another example and i'm knowing better this time. – Emilio López Dec 02 '14 at 12:18
Also, `$addToSet` does not affect existing duplicate elements. – Deepak Sah Dec 17 '19 at 06:09

score 34 · Answer 2 · edited Jun 14 '19 at 05:35

34

$push - adds items in the order in which they were received. Also you can add same items several times.

$addToSet - adds just unique items, but order of items is not guaranteed.

If you need to add unique items in order, you can group and add elements via $addToSet, then $unwind the array with elements, $sort by items, and then do $group again with $push items.

edited Jun 14 '19 at 05:35

FishSaidNo

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answered Dec 29 '17 at 10:15

Oleksandr Lukichov

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score 3 · Answer 3 · answered Mar 01 '19 at 09:49

Along side the differences that others have mentioned

$push: Appends an object to an array
$addToSet: Adds an object to an array if it does not exists

There is a difference in replication. (This can be seen if by taking a look at local.oplog.rs)

$push operations (that actually modified the array) are replicated as $set.items.INDEX: item
$addToSet operations (that actually modified the array) are replicated as $set.items: [THE_ENTIRE_ARRAY]

If you are dealing with large arrays, then the difference might be significant

So while something like (the typical use case to maintain unique array)

db.items.updateOne(
  {_id: 'my-id', 'items': {'$ne': 'items1'},
  {'$push': {
    'items': 'item1',
  }}
)

db.items.updateOne(
  {_id: 'my-id'},
  {'$addToSet': {
    'items': 'item1',
  }}
)

might end up with the same resulting document, there is a difference in the replicated operation.

score 2 · Answer 4 · edited Aug 28 '17 at 10:39

2

$addToSet and $push does the same thing, however $push just pushes any item disregarding the duplication causing redundancy. The former pushes only unique items, no duplication.

edited Aug 28 '17 at 10:39

dat3450

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answered Jul 03 '17 at 02:31

Remario

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score 2 · Answer 5 · answered Sep 04 '17 at 15:43

2

As the name suggest $addToSet (set) wont allow duplicates while $push simply add the element to array

answered Sep 04 '17 at 15:43

Vipul Pandey

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score 0 · Answer 6 · answered Jul 15 '18 at 07:46

$push: Inserts the value to an array in the resulting document. eg;

db.mycol.aggregate([{$group : {_id : "$by_user", url : {$push: "$url"}}}])

$addToSet: Inserts the value to an array in the resulting document but does not create duplicates. eg;

db.mycol.aggregate([{$group : {_id : "$by_user", url : {$addToSet : "$url"}}}])

MongoDb: Difference between $push/$addtoset

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