Fixed problem populating several fields

We were having this error with version `3.1.2`:

```
Caught exception: MissingSchemaError: Schema hasn't been registered for model "user".
Use mongoose.model(name, schema)
    at NativeConnection.Connection.model (/Users/gimenete/projects/DocChat/node_modules/mongoose/lib/connection.js:625:11)
    at populate (/Users/gimenete/projects/DocChat/node_modules/mongoose/lib/model.js:2085:24)
    at Function.Model.populate (/Users/gimenete/projects/DocChat/node_modules/mongoose/lib/model.js:2050:5)
    at Object.cb (/Users/gimenete/projects/DocChat/node_modules/mongoose/lib/query.js:1021:16)
    at Object._onImmediate (/Users/gimenete/projects/DocChat/node_modules/mongoose/node_modules/mquery/lib/utils.js:137:16)
    at processImmediate [as _immediateCallback] (timers.js:345:15)
```