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Databases Mongo Basics Working With Collections Managing Collections

Brendan Whiting
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Brendan Whiting
Front End Web Development Techdegree Graduate 84,738 Points

Why is one "_id" and the other "_id_"?

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

And then when I try and delete _ id _ it refers to it without the 2nd underscore as _ id:

> db.posts.dropIndex('_id_');
{
    "nIndexesWas" : 1,
    "ok" : 0,
    "errmsg" : "cannot drop _id index",
    "code" : 72
}

What's this underscore business?

1 Answer

Ken Alger
STAFF
Ken Alger
Treehouse Teacher

Brendan;

Great question. As you learned, MongoDB automatically generates a field for us called _id. By default it is assigned a type of ObjectId, which is a 12-byte BSON type. Indexes are vital to performance in MongoDB and the database automatically generates an index on the _id field and assigns the index a name _id_. This is a built in index and cannot be dropped, as it sounds like you discovered. Further it's name cannot be altered because there currently is not a way to modify an index in MongoDB, they have to be dropped and recreated.

Post back if you have further questions.

Happy coding,
Ken

Ken Alger
Ken Alger
Treehouse Teacher

_id is the field name. _id_ is the name of the index.

Why are they named that way... because that is what the developers at MongoDB decided. :smile:

Happy coding,
Ken