Rails - has_many :through - Create new record and association in one form

Question

I am currently struggling with a has_many :through association in my project. The basics of the association are as follows:

class Course < ActiveRecord::Base
    has_many :contents
    has_many :topics, :through => :contents
end

class Topic < ActiveRecord::Base
    has_many :contents
    has_many :courses, :through => :contents
end

class Content < ActiveRecord::Base
    belongs_to :courses
    belongs_to :topics
end

probably could not be more basic..

So the idea is that a user can create many courses, and also create many topics. Topics are associated with courses via the Content class. Doing it this way means a user could create a number of topics and associate them with a number of courses. Great, saves the user typing out the lot again when course topics overlap. This is all fine and dandy.

However, I want the user to first create a Course and then from there create a number of new topics for that course. Seems logical to me.

My issue is I am struggling to get my head around the best way to do this?

I could scrap the :through association and have a basic topic belongs_to course association as this would do what i want but at the expense of the extra functionality I want.

I am thinking along the lines of a form_for Topic with fields_for content? I can't help thinking this is a common problem with a common answer but cant find the answer on the interweb. Maybe its my wording. Hope this make complete sense to someone....?

Thanks in advance

Answer 1

I'll write this from a somewhat general perspective (but using the models you described), because it is not just relevant to the situation you described, but relevant to any time you're creating a new association in a Many-to-Many relationship using has_many :through.

In rails, here is a simple example to create a new Topic object, which is associated with a course:

@course.topics << Topic.new(params[:topic])

The above assumes you have previously already loaded up your Course object and stored it in @course. It also assume that the Topic data is coming from a form, and stored in the parameter map under the :topic key.

If you examine your log when this portion executes, because you set up your associations correctly, you should see two insert statements: INSERT INTO "topics"... and INSERT INTO "contents"....

There are other ways (some far more roundabout than others) to do this, but I believe this is the most straightforward.

Let me know if that makes sense.