mongodb why do we need getSisterDB

Question

While playing with mognodb console help, I found a db.getSisterDB() method.

And I am curious what is the purpose of this method. Looking through mongodb documentation and a quick google search did not yield satisfactory results.

By typing db.getSisterDb.help generates an error and typing db.getSisterDB gives the following definition of this method:

function ( name ){
    return this.getMongo().getDB( name );
}

which suggests that this is just a wrapper around getDB. My suggestion that it is used in to access databases in a replica set, but I would like to listen to a person who can give me a more thorough explanation.

Answer 1

In the shell, db is a reference to the current database. If you want to query against a different DB in the same mongod instance, the way to get a proper reference to it would be to use this method (which has an alias, more gender neutral getSiblingDB).

If you wanted to use the longer syntax, you could: db.getMongo().getDB(name) gets you the same thing as db.getSiblingDB(name) or db.getSisterDB(name) but the former is longer to type.

All of the above work the same way in standalone mongod as well as replica sets (and sharded clusters).