How can I get url for paperclip image in to_json

Question

I believe the easiest way for you to accomplish this will be to create a method in your object to return the avatar URL.

class Model < ActiveRecord::Base
    ...

    def avatar_url
        avatar.url(:medium)
    end

    ...
end

This will then allow you to use the methods option when calling to_json with a simple method that does not require any parameters:

respond_to do |format|
   render :json => @model.to_json(:only => [:id,:name,:homephone,:cellphone], :methods => [:avatar_url])
end

Which should yield you an output along these lines:

{"id" => 1, "name" => "Cool model", "homephone" => 1234567890, "cellphone" => 0987654321, "avatar_url" => "www.coolsite.com/this_avatars_path"}

See these for reference:

Ruby to_json :methods arguments

http://apidock.com/rails/ActiveRecord/Serialization/to_json