I have this sample document:

```
{
    "_id" : ObjectId("4f98fd1df2699a2f8a000003"),
    "comments" : "Foo bar",
    "location" : "Somewhere",
    "user_id" : ObjectId("4f98fd1df2699a2f8a000001")
}

```

When I retrieved the data using controller: 

```
respond_to :json

def index
  respond_with Comment.all
end

```

It returns JSON:

```
[{
   "_id": "4f98fd1df2699a2f8a000003",
    "comments": "Foo bar",
    "location": "Somewhere",
    "user_id": "4f98fd1df2699a2f8a000001",
}]

```

My question is how can I include the fields of User class on the response easily? 

```
class User
    include Mongoid::Document

    field :username
    field :first_name
    field :last_name
end

```