How to render an array.to_json with :include option in Rails?

In my controller I have :

def index
  # Array of Task objects
  @tasks = Task.get_tasks(current_user, params)

  respond_to do |format|
    format.html

    # Sends correct JSON开发者_JS百科 but not including the 'author' object in it
    # format.json { render :json => @tasks.to_json(:include => [:author]) }

    # With this the JSON look correct but is interpreted as a string in the JavaScript code
    format.json { render :json => @tasks.map { |e| e = e.to_json(:include => [:author]) } }
  end
end

Do you know any 'clean' solution to properly pass the :include option when rendering an array converted to JSON ?

EDIT

I am using MongoDB

EDIT (2)

I updated from mongoid (2.0.1) to mongoid (2.0.2) and it works. Sorry for the trouble.

The to_json is redundant. I just tested it and with works with some similar code here using the syntax:

format.json { render :json => @tasks, :include => [:author] }

This is rails 3.0.7. This is also assuming that author is set as a belongs_to of Task.