About: Maker of things, giver of high-fives 🖐
Let's say I have a model for a thing called "Chapter". I want to be able to reference the next chapter and the previous chapter for each chapter. How would you go about setting this up?
So what I'm looking for is something like this:
c = Chapter.first
c.next_chapter
c.next_chapter should return a chapter object that is the next chapter.
Thanks Miguel for the tip on setting up the method :) this is super helpful.
guides.rubyonrails.org/association...
# in migration
def change
add_reference :chapters, :previous_chapter, foreign_key: { to_table: :chapters }
end
# chapter.rb
belongs_to :previous_chapter, class_name: ‘Chapter’
has_one :next_chapter, class_name: ‘Chapter’, foreign_key: :previous_chapter_id
Thanks Brian for the reply!
has_one :next_chapter, class_name: ‘Chapter’, foreign_key: :previous_chapter_id
Why is the foreign_key the previous_chapter_id and not something like next_chapter_id?
Because that’s a reference to the database column in your migration. That line basically says, “When the :next_chapter method is called, return the chapter that lists this one as its :previous_chapter.”
You could set up the migration to store the next chapter instead of the previous one. It’s the same process. I just thought it makes more sense this way because when you create a chapter, the previous one probably exists already, whereas the next one might not.
Ahhh clever, thanks Brian for the help. I implemented this method and it worked as you described it :)
Assuming a Book class exist and it has many chapters, and also assuming that the chapter has a position number stored in the database, I'd do something like:
class Chapter < ApplicationRecord
belongs_to :book
def next_chapter
book.chapters.find_by(position: position + 1)
end
def previous_chapter
book.chapters.find_by(position: position - 1)
end
end
I would go with Brian’s solution then. The other solutions work, but I would prefer to have a strong reference, in this case a foreign key stored in the Database.
Yup that's the solution that I ended up going with. Thanks Christopher for the help!
I'd question whether a method like this should sit in the model (should it actually have knowledge of its sibling persisted objects? Usually not)
Depending on your context, making a small class/service/whatever which determines position might be more suitable (and easier to test/maintain).
in your app/models/chapter.rb
Explanation:
The first line checks if the chapter that you called
next_chapteron is the last one and if so, it just returns itself (You can change this to an error message or whatever you want). And if that's not the case, then we call find on chapter, which takes the Chapter ID as an argument and we just add one to the current ID. And voilà: Now you can callnext_chapteron yourChapterinstances