Habtm Associations In Rails : Collecting And Counting The Categories Of A Model's Children

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

Solution 1:

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.

classStore < ActiveRecord::Base
  has_many :books
  has_many :categories, through::booksend

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
    }
  endend

Courtesy of @jakub-kosiński

Solution 2:

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')