Question

That title sounds weird, so let me try to explain what it is I’m trying to accomplish.

Let’s say I have a Category model, and the structure looks a little like this:

categories

id

name

parent_id

---

I also have a model for articles

articles

id
category_id

---

If I have a category named “News” with id: 1, and then a subcategory for it named “World News” with the id: 2 and parent_id: 1, if I have an article with category_id: 1 and another article with category_id: 2, I would like to be able to do something like:

category = Category.find 2 # This is the subcategory
category.articles # => Shows just the article belonging to that category

And

category = Category.find 1 # This is the parent category
category.articles # => Shows BOTH articles, the one for the child and the one for parent

Does that make sense?

Answer 1

What you are looking for is a tree structure for your categories. Using parent_id is the most obvious way to implement it, but it gets complicated when you want to query children.

A better solution is to use a nested set, which adds a bit of complication on insert/delete but pays off with very simple queries.

You can do the research to implement this, but as with a lot of things in rails, it’s a solved problem. Use awesome_nested_set which will take care of all of this for you.

Once you have installed the gem, create this migration:

def up
  add_column :categories, :lft, :integer
  add_column :categories, :rgt, :integer

  Category.reset_column_information!
  Category.rebuild!
end

def down
  remove_column :categories, :lft
  remove_column :categories, :rgt
end

Then add this line to your model:

class Category < ActiveRecord::Base
  acts_as_nested_set

  # ...
end

You can use a query like this to get all articles:

category = Category.find(1) # Or any category id

category.self_and_descendants.articles