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"
    }
]

Answer 2

I was confused by exactly the same error today, and later figured it out. It was because I removed a indexed property from a mongoose schema, but did not drop that property from the mongodb index. The error message is infact that the new document has an indexed property whose value is null (not in the json).