mongodb - mongo _id field duplicate key error

Question

I have a collection with the _id field as a IP with type String.

I'm using mongoose, but here's the error on the console:

$ db.servers.remove()

$ db.servers.insert({"_id":"1.2.3.4"})

$ db.servers.insert({"_id":"1.2.3.5"}) <-- Throws dup key: { : null }

Answer 1

Likely, it's because you have an index that requires a unique value for one of the fields as shown below:

> db.servers.remove()
> db.servers.ensureIndex({"name": 1}, { unique: 1})
> db.servers.insert({"_id": "1.2.3"})
> db.servers.insert({"_id": "1.2.4"})
E11000 duplicate key error index: test.servers.$name_1  dup key: { : null }

You can see your indexes using getIndexes() on the collection:

> db.servers.getIndexes()
[
    {
        "v" : 1,
        "key" : {
                "_id" : 1
        },
        "ns" : "test.servers",
        "name" : "_id_"
    },
    {
        "v" : 1,
        "key" : {
                "name" : 1
        },
        "unique" : true,
        "ns" : "test.servers",
        "name" : "name_1"
    }
]