Question

Normally in html we will use Model.field.url(:thumb) inside image tag, How to do it on json, especially with hash_secret.

Answer 1

In your model add the following to get the url (I believe this also works with hashing):

def photo_url_thumb
    photo.url(:thumb)
end 

And then you can output json like this:

 format.json { render :json => @model.photo_url_thumb }