Rails通过连接表获得关联计数
这个问题是Rails中HABTM协会的一个分支：收集和计算模型儿童的类别 。
鉴于：
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 任务：
给定商店，列出每个类别的书籍数量。
Store.first.books_per_category
期望的输出：
[ { name: 'mystery', count: 5 }, { name: 'fantasy', count: 6 } ]
然而，每家商店可能拥有大量的书籍和类别 。
我正在尝试创建一个单一的高性能查询，该查询只获取名称列和每个与商店关联的不同类别的书籍计数，而不将书籍加载到内存中。
我到目前为止尝试过：
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
数据库架构：
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
您可以使用链select和group来汇总每个类别的书籍数量。 您的books_per_category方法可能如下所示：
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
这将产生以下SQL查询：
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
你想要在Categories对象上创建一个方法（或范围），就像这样。
Category.joins(:books).select('categories.*, COUNT(books.id) as book_count').group('categories.id')
生成的对象现在将具有类别实例的每个属性，并响应方法book_count ，该方法返回具有该实例类别id的书籍数量。 值得一提的是，这将省略任何没有与之相关的书籍的类别。 如果要包含这些，则需要将查询更新为以下内容：
Category.joins('LEFT OUTER JOIN books_categories on books_categories.category_id = categories.id').select('categories.*, COUNT(books_categories.book_id) as book_count').group('categories.id')