MongoDb：$push/$addtoset 之间的区别
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MongoDb: Difference between $push/$addtoset
提问by Emilio López
I read the documentation in the MongoDb and I used a simple proves and I only look that:
Push is sorting the array but addtoSetisn't it.
我阅读了 MongoDb 中的文档，并使用了一个简单的证明，我只看：推送正在对数组进行排序，但addtoSet不是。
For me visually is the same, I don't know the difference.
对我来说视觉上是一样的，我不知道有什么区别。
Could anybody explain me the difference?
有人能解释一下区别吗？
Another think if it could be in spanish or in a simple english, i'll aprecite it.
另一个认为，如果它可以是西班牙语或简单的英语，我会很感激。
回答by cubbuk
$addToSetdo not add the item to the given field if it already contains it, on the other hand $pushwill add the given object to field whether it exists or not.
$addToSet如果项目已经包含它，则不要将它添加到给定的字段中，另一方面$push，无论是否存在，都会将给定的对象添加到字段中。
{_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: {item:2}}); // this will update the document. new document {_id: "docId", items:[1,2,2]}
回答by Oleksandr Lukichov
$push- adds items in the order in which they were received. Also you can add same items several times.
$push- 按照收到的顺序添加项目。您也可以多次添加相同的项目。
$addToSet- adds just unique items, but order of items is not guaranteed.
$addToSet- 只添加唯一的项目，但不保证项目的顺序。
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.
如果您需要按顺序添加唯一项，您可以通过 $addToSet 对元素进行分组和添加，然后用元素 $unwind 数组，按项进行 $sort，然后再用 $push 项执行 $group。
回答by Allan Lei
Along side the differences that others have mentioned
除了其他人提到的差异
$push: Appends an object to an array$addToSet: Adds an object to an array if it does not exists
$push: 将对象追加到数组$addToSet: 如果对象不存在，则将其添加到数组中
There is a difference in replication. (This can be seen if by taking a look at local.oplog.rs)
复制是有区别的。（这可以通过查看来了解local.oplog.rs）
$pushoperations (that actually modified the array) are replicated as$set.items.INDEX: item$addToSetoperations (that actually modified the array) are replicated as$set.items: [THE_ENTIRE_ARRAY]
$push操作（实际上修改了数组）被复制为$set.items.INDEX: item$addToSet操作（实际上修改了数组）被复制为$set.items: [THE_ENTIRE_ARRAY]
If you are dealing with large arrays, then the difference might be significant
如果您正在处理大型数组，那么差异可能很大
So while something like (the typical use case to maintain unique array)
因此，虽然类似（维护唯一数组的典型用例）
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.
可能最终得到相同的结果文档，复制操作存在差异。
回答by Vipul
As the name suggest $addToSet (set) wont allow duplicates while $push simply add the element to array
顾名思义 $addToSet (set) 不允许重复，而 $push 只是将元素添加到数组
回答by Remario
$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.
$addToSet 和 $push 做同样的事情，但是 $push 只是推送任何项目，而忽略导致冗余的重复。前者只推送唯一的项目，没有重复。
回答by Hasan
$push: Inserts the value to an array in the resulting document. eg;
$push：将值插入到结果文档中的数组中。例如;
db.mycol.aggregate([{$group : {_id : "$by_user", url : {$push: "$url"}}}])
$addToSet: Inserts the value to an array in the resulting document but does not create duplicates. eg;
$addToSet：将值插入到结果文档中的数组中，但不创建重复项。例如;
db.mycol.aggregate([{$group : {_id : "$by_user", url : {$addToSet : "$url"}}}])
