HABTM associations in Rails : collecting and counting the categories of a model's children

Question

I have a has_and_belongs_to_many relationship setup. It looks like this:

• books have_and_belong_to_many categories
• categories have_and_belongs_to_many books
• a store has_many books
• a book belongs_to a store

I'm trying to show how many books in each store belong to each category. So my view would show Store X has 200 books and 80 of them are mystery, 60 are non fiction, etc.

I have been trying out a bunch of different ways of doing this, but no success so far. I think I'm starting in the wrong place. Any direction would be much appreciated.

Thanks

This is Rails 4 and psql by the way.

Answer 1

Generally, instead of using Rails' built-in 'has_and_belongs_to_many' method, it is better practice to use a join table. In this setup, you have three tables:

• Books
• Categories
• BookCategories

The BookCategories (or whatever you decide to call it) is a join table that belongs_to both Books and Categories and has a Foreign ID of each. You would then use Rails' "has_many :through" to link the Books and Categories.

The store would have a 'has_many' relationship with books. With the prior relationship setup right, you can then use this method to get the count for a store for a particular category:

Store.books.where(category:'Mystery')

Answer 2

Provided that you have a books_categories join table you can add a has_many :categories, through: :books association to which links stores and categories through books .

class Store < ActiveRecord::Base
  has_many :books
  has_many :categories, through: :books
end

That's the easy part. Now lets get each category and the books count (revised):

def books_per_category
  categories.select('categories.id, categories.name, count(books.id) as count').group('categories.id, categories.name').map do |c|
    {
      name: c.name,
      count: c.count
    }
  end
end

Courtesy of @jakub-kosiński