Depending on your query needs you can put everything in the user document:
{name:"Joe"
,roles:["Admin","User","Engineer"]
}
To get all the Engineers, use:
db.things.find( { roles : "Engineer" } );
If you want to maintain the roles in separate documents then you can include the document’s _id in the roles array instead of the name:
{name:"Joe"
,roles:["4b5783300334000000000aa9","5783300334000000000aa943","6c6793300334001000000006"]
}
and set up the roles like:
{_id:"6c6793300334001000000006"
,rolename:"Engineer"
}