grails-documentation-mongodb/src/docs/guide/3.5 Dynamic Database or Collection Switching.gdoc

In addition to storing dynamic attributes, as of version 1.3.0 of the plugin you can also switch which database and/or collection to persist to at runtime. 

For example:

{code}
Person.withDatabase("administrators") {
    new Person(name:"Bob").save()
}
{code}

The above example will save a @Person@ instance to the 'administrators' database. The database is used for the scope of the closure. You can switch database for the scope of the active session:

{code}
Person.useDatabase("administrators") 
new Person(name:"Bob").save()
{code}

In addition, there are equivalent @withCollection@ and @useCollection@ methods for switching collection at runtime.