Error in rendering json

Hello,

in my controller I use following code to render json.

  render :json => @product, :callback => params[:callback]

I would expect (for example) following to be rendered:

  callback({"name":"Computer","price":500})

Instead, it renders:

  callback([{"name":"Computer","price":500}])

Notice the square brackets which disables $.parseJSON (jQuery) from
parsing it.
Don't you know what am I doing wrong? How can I get rid of those
brackets? When I tried ActiveSupport::JSON.encode() there were no
square brackets.

Thank you very much,
Jakub Svehla