MongoDB-为什么_id索引不对重复的条目引发错误? [英] MongoDB - Why does the _id index not throwing an error on duplicate entries?

问题描述

我对NoSQL数据库完全陌生，目前正在使用MongoDB.

I'm completely new to NoSQL databases and I'm working currently with MongoDB.

我试图理解为什么upserting a duplicate _id文档时默认的_id索引不引发错误.

I'm trying to understand why the default _id index does not throw an error, when upserting a duplicate _id document.

如文档中所述_id默认情况下是唯一索引

As stated in the docs _id is an unique index by default

(尽管此处未显示唯一标志.)

(although it doesn't show the unique flag here..)

> db.foo.getIndexes();
[
    {
        "v" : 1,
        "key" : {
            "_id" : 1
        },
        "name" : "_id_",
        "ns" : "test.foo"
    }
]
>

因此，当upserting文档(以空集合开始)时，
如果先插入它，然后似乎忽略"它.

So when upserting the document (started with an empty collection),
if first inserts it and then seems to "ignore" it.

> db.foo.update({ _id: 'doe123'}, { _id: 'doe123', name: 'John Doe'}, { upsert: true});
WriteResult({ "nMatched" : 0, "nUpserted" : 1, "nModified" : 0, "_id" : "doe123" })

> db.foo.update({ _id: 'doe123'}, { _id: 'doe123', name: 'John Doe'}, { upsert: true});
WriteResult({ "nMatched" : 1, "nUpserted" : 0, "nModified" : 0 })

因此，我尝试了另一件事，并在名称"上创建了unique index.

So I've tried another thing and created an unique index on "name".

名称重复的结果:

> db.foo.update({ _id: 'foo456'}, { _id: 'foo456', name: 'John Doe'}, { upsert: true});
WriteResult({
    "nMatched" : 0,
    "nUpserted" : 0,
    "nModified" : 0,
    "writeError" : {
        "code" : 11000,
        "errmsg" : "E11000 duplicate key error collection: test.foo index: name_1 dup key: { : \"John Doe\" }"
    }
})

为什么我不在重复的_id上遇到这种错误?

Why am I not getting this kind of error on a duplicate _id?

编辑:我正在使用MongoDB v.3.2.3

EDIT: I'm using MongoDB v.3.2.3

推荐答案

在第一种情况下，没有理由显示重复的索引错误，因为它只是尝试更新同一记录的_id和name字段具有相同的值.

There is no reason to show duplicate index error in the first case as it is just trying to update the _id and name fields of the same record with the same value.

如果可以尝试

  db.foo.update({ _id: '1098'}, { _id: 'doe123', name: 'John Doe'}, { upsert: true});

您将收到错误消息，因为查询正尝试使用具有某些_id值的其他_id更新记录.

you will get error, as query is trying to update record with different _id with some existing _id value.

在第二种情况下，您首先使用name字段创建了一条记录，然后尝试在另一条记录中更新相同的名称，由于name是唯一索引，因此会出现错误.

In second case, you created a record first with name field and then you are trying update the same name in another record, which will give error as name is unique index.

-

如果您正在尝试

 db.foo.insert({ _id: 'doe123', name: 'John Doe'});

会给您错误，因为在这种情况下，您试图插入一条已经存在的记录，即_id是唯一的，并且您试图再创建一个具有相同_id值的记录.

will give you the error, as in this case you are trying to insert a record which is already present i.e. _id is unique and you are trying to create one more record with same _id value.

upsert:true:-如果upsert为true，并且没有文档符合查询条件，则update()将插入单个文档.

upsert:true :- if upsert is true and no document matches the query criteria, update() inserts a single document.

如果upsert为true，并且存在符合查询条件的文档，则update()将执行更新.

If upsert is true and there are documents that match the query criteria, update() performs an update.

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