Question

Let’s say that each Product has a category. I want to ask the Users to select several categories that the user is interested in, and find the Products that have the same category. This is similar to what Quora, Stumbleupon, and Pinterest all do.

What would be the best way to set this database structure in Rails? Should I create 3 tables: User, Product, and Category, and make the relations
User has many Categories & Product has many Categories?

The problem I see with this is doesn’t it create, rather than reference, a new instance of Categories to each row of Users and Products?

*extra: What if I wanted subcategories? For example, if the user chose Technology, it could further ask to choose between web dev, mobile dev, hardware, etc.

Answer 1

You could do that kind of ‘recommendation’ pretty easily.

Something like this should work (N.B.: I did not test this code, but it is right in spirit):

def recommended_products
  joins(:categories, :products).where("product_id not in (?)", self.products)
end

Explanation of each bit:

joins(:categories, :products): this does a SQL join of users, products, and categories. This gives you a ‘table’ where each user-product-category combination is in it’s own row.

.where("product_id not in (?)", self.products): adds a SQL where clause to filter out all the rows that have products in the current user’s list of products.

The associations are not a problem. They don’t create any new instances by themselves, only if you write code that creates new instances yourself.

As for sub categories, I think you’ll do better to make that it’s own question, as it’s easily a whole post in itself.