Update attribute in associated model in Rails when parent is updated - ruby-on-rails

Question

I have two models User and Assignment. Whenever User is updated I want to update the url attribute in Assignment.
How do I do this?
class User
has_many :assignments
...
end
class Assignment
belongs_to :user
before_save :set_url
def set_sandbox_url
language = 'www'
project = 'example'
base_url = "https://#{language}.#{project}.org/"
sandbox_url = "#{base_url}/User:#{user.username}/#{article_title}"
end

Answer 1

I agree with Tamer Shlash that there is not really a benefit in storing this URL in the database because it can be easily generated each time you need it.
But apart from that, I would like to answer your question. Your callback to regenerate the URL doesn't work for various reasons. First, you want to update the URL when the user changes therefore the user needs to have a callback defined. Second, the naming is not correct. The callback as it is currently written would try to run a set_url method but the method is actually called set_sandbox_url. And third, sandbox_url = will just assign the new URL to a local variable sandbox_url but it would not update the instance variable #sandbox_url.
I would do something like this:
# in app/models/user.rb
after_save :update_assignment_url
private
def update_assignment_url
assignments.each(&:update_url) if username_previously_changed?
end
# in app/models/assignments.rb
def update_url
language = 'www'
project = 'example'
base_url = "https://#{language}.#{project}.org/"
sandbox_url = "#{base_url}/User:#{user.username}/#{article_title}"
update!(sandbox_url: sandbox_url)
end
Note: because you build the URL by simply concatenating strings I suggest making sure that these strings (especially values provided by the user like username and article_title) only include characters that are valid in an URL (for example by using String#parameterize).
You might want to read about Dirty Attributes too which provided the used username_previously_changed? method.