[SOLVED] Rails get count of association through join table

Issue

This question is an offshoot from HABTM associations in Rails : collecting and counting the categories of a model's children.

Given:

class Category < ActiveRecord::Base
  has_and_belongs_to_many :books
  validates_uniqueness_of :name
end

class Book < ActiveRecord::Base
  has_and_belongs_to_many :categories
end

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

The task:

Given a store, list the number of books per category.

Store.first.books_per_category

desired output:

[ { name: 'mystery', count: 5 }, { name: 'fantasy', count: 6 } ]

However each store may have an enormous amount of books and categories.

I am trying to create a single performant query which only gets the name column and a count of the Books for each distinct Category associated with a Store, without loading books into memory.

I have tried so far:

class Store < ActiveRecord::Base

  # Will load each book into memory
  def books_per_category
    categories.eager_load(:books).map do |c|
      {
          name: c.name,
          count: c.books.size # Using size instead of count is important since count will always query the DB
      }
    end
  end

  # will query books count for each category.
  def books_per_category2
    categories.distinct.map do |c|
      {
          name: c.name,
          count: c.books.count
      }
    end
  end
end

Database schema:

ActiveRecord::Schema.define(version: 20150508184514) do

  create_table "books", force: true do |t|
    t.string   "title"
    t.datetime "created_at"
    t.datetime "updated_at"
    t.integer  "store_id"
  end

  add_index "books", ["store_id"], name: "index_books_on_store_id"

  create_table "books_categories", id: false, force: true do |t|
    t.integer "book_id",     null: false
    t.integer "category_id", null: false
  end

  add_index "books_categories", ["book_id", "category_id"], name: "index_books_categories_on_book_id_and_category_id"
  add_index "books_categories", ["category_id", "book_id"], name: "index_books_categories_on_category_id_and_book_id"

  create_table "categories", force: true do |t|
    t.string   "name"
    t.datetime "created_at"
    t.datetime "updated_at"
  end

  create_table "stores", force: true do |t|
    t.string   "name"
    t.datetime "created_at"
    t.datetime "updated_at"
  end
end

Solution

You can use chain select and group to aggregate the count of books for each category. Your books_per_category method may look like this:

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

This will produce the following SQL query:

SELECT categories.id, categories.name, count(books.id) as count 
  FROM "categories" 
  INNER JOIN "books_categories" ON "categories"."id" = "books_categories"."category_id" 
  INNER JOIN "books" ON "books_categories"."book_id" = "books"."id" 
  WHERE "books"."store_id" = 1 
  GROUP BY categories.id, categories.name

Answered By – Jakub KosiƄski

Answer Checked By – Gilberto Lyons (BugsFixing Admin)

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