Rails%203%20In%20Action

Classes outside your control
It’s time this method moved out of your imagination and into your engine. This
method needs to return the last post for your topic, and because the topic_id lives on
the posts table and you’re wanting only one of these posts, this is a great opportunity
to use a has_one association.
This association needs to find the chronological last post for your topics; you can
do this by defining this association in app/models/forem/topic.rb like this:
has_one :last_post, :class_name => "Forem::Post",
:order => "created_at DESC"
The class_name option you’ve used before; it tells Rails that objects of this association
are of that class, rather than the inferred class, which is a constantized version of the
association name. In this case, that would be LastPost. The :order option, however,
will order the posts by the created_at field in reverse chronological order. The
has_one method will then execute this query and limit the results to one, returning
only the first post.
With the association defined, you can re-run your test and see it passing:
1 example, 0 failures
You’ve now got a test that covers that a user can see the posts counter and the last post
information on the topics page for a topic, which proves that this feature is working.
Let’s run all the RSpec tests to make sure that everything’s working with bin/rspec
spec:
6 examples, 0 failures
Great, everything’s good. Time for a commit:
git add .
git commit -m "Add last post information to topics index"
This has been quite a long section and we’ve covered a lot of ground. The purpose of
this section was to demonstrate how you could reference classes outside your control,
such as those in the application or found in other engines. We opted for configuration
options for some of the options, and a module for others.
You then asked users to authenticate before they could create new topics and
posts, and after that you linked your engine’s classes’ objects to the objects from an
application’s class. This is kind of a big deal, as it shows that an engine is able to interact
with an application without extravagant modification of the application’s code.
Finally, you fixed up your topics index view to display information from these
linked-in classes.
This has been only the beginning of linking your application and engine. In the
final sections of this chapter, you’ll see how you can release your engine as a gem and
how you can integrate it into your application so that the users of Ticketee have a
forum system they can use.
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