How to make more complex queries?

Hi, I want to completely change the structure of my database (mongodb), so the four methods provided by the API will not suffice. How do I use the mongodb API from within migrations?
I am currently stuck with this:

```
exports.up = (db, callback) ->
db.connection._db.collection('users').find().toArray((err, arr) ->
console.log err
console.log arr
callback()
)
```

Which yields:

```
{ [MongoError: Connection Closed By Application] name: 'MongoError' }
null
```
##

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