check for multiple attributes existence

The $exists method is not special. It just checks the documents that have the field you specified. You can use it like you would use any other operator in MongoDB.

If you need to check multiple attributes, you just need to add them to the query:

db.things.find({ 
    otherInfo: { $exists: true },
    foo: { $exists: true },
    bar: { $exists: true },
    a : 123,
    b : 234
    /* ... */
});