ruby on rails - Derive ActiveRecord model field from id -

I am writing an application to parse my iTunes xml library in a database in Ruby and I'm having a problem .. I have a wrapper model in which there is a field, file name, which I need to get from another field, album_id in that field

file name = Album_ID 10 '0 and jpg extension Is frustrated by ...

but field ALBUM_ID, as the name suggests Security is a foreign key to another model, album. I only get ALBUM_ID while saving the album ...

Is there a way to get my file name folder to get ALBUM_ID using callback, I do not know, maybe BEFORE_SAVE or AFTER_SAVE?

- == UPDATE == -

Make me clear ... I have a field in the table name FILENAME ... this field The file name of the cover is in this format

ID 1 = 0000000001.jpg for album with id = 5600000056.jpg

Cover the album model There is a HAS_MANY relationship with the model ... When I save the album, I have to get the saved value in ALBUM_ID in the cover model and create a string in that format and Must use the file name field.

I am doing, in the second part, to copy jpgs to cover the broom in another library folder in another library ... This is why I need to rename the files

As I have understood that you have a cover model By using related_to the album model, album_id field.

When creating a album , you can easily .build

  album = album.new (: name = & "Foo",: Artist => "Bar", ...) album.cover = Cover.build (: filename => "filename.png") # Save when you are saving the album  

Now that you album is automatically generated from cover with correct album_id .

Second Solution

After reading your post again, the only purpose of your cover model is to generate a JPG with the album ID.

If you have jpg 00000000001 .jgp , why would not you name it in a helper or idea?

show.html.erb

  & lt; Img url = "0000000000 & lt;% = @ album.id%> .jpg" / & gt;