Rails 4, has_many through association - finding associated objects

问题

I'm wondering why Item.where(category: x) isn't working for me. I expect this statement to return all items which are categorized under category x. Please see below my associations:

class Category < ActiveRecord::Base
    has_many :categorizations
    has_many :items, :through => :categorizations
end

class Item < ActiveRecord::Base
    has_many :categorizations
    has_many :categories, :through => :categorizations
end

class Categorization < ActiveRecord::Base
    belongs_to :item
    belongs_to :category
end

Category.find(1).items returns all items for that category. Please see below, the byebug console output which demonstrates my point further. It seems like the SQl that is generated is looking for a category column in the Item table - which of course doesn't exist. Any ideas?

    1: class ItemsController < ApplicationController
    2:   # Returns full list of items
    3:   def index
    4:     @items = Item.all
    5:      byebug
=>  6:   end
    7: 

(byebug) Item.where(category: 1)
  Item Load (0.6ms)  SELECT "items".* FROM "items" WHERE "items"."category" = 1
#<Item::ActiveRecord_Relation:0x007fb5d1a37f08>
(byebug) Category.find(1).items
  Category Load (0.7ms)  SELECT  "categories".* FROM "categories" WHERE "categories"."id" = $1 LIMIT 1  [["id", 1]]
  Item Load (1.7ms)  SELECT "items".* FROM "items" INNER JOIN "categorizations" ON "items"."id" = "categorizations"."item_id" WHERE "categorizations"."category_id" = $1  [["category_id", 1]]
#<ActiveRecord::Associations::CollectionProxy [#<Item id: 1, title: "Gorgeous Cotton Pants", description: "Dolor dicta suscipit aut cupiditate quia officiis ...", price: 73960, status: 0, published_date: "2016-07-14 05:35:49", created_at: "2016-07-17 05:15:07", updated_at: "2016-07-17 05:15:07", seller_id: 1>, #<Item id: 5, title: "Sleek Marble Shoes", description: "Qui mollitia corporis qui placeat. Reiciendis ea s...", price: 35146, status: 0, published_date: "2016-07-14 05:45:02", created_at: "2016-07-17 05:15:07", updated_at: "2016-07-17 05:15:07", seller_id: 1>, #<Item id: 7, title: "Rustic Concrete Lamp", description: "Sit odio non exercitationem. Atque non sapiente vo...", price: 82016, status: 2, published_date: "2016-07-13 00:00:00", created_at: "2016-07-17 05:15:07", updated_at: "2016-07-17 05:15:07", seller_id: 1>, #<Item id: 10, title: "Awesome Wooden Table", description: "Possimus consequatur nulla. Quidem molestiae volup...", price: 59519, status: 2, published_date: "2016-07-09 00:00:00", created_at: "2016-07-17 05:15:07", updated_at: "2016-07-17 05:15:07", seller_id: 1>, #<Item id: 12, title: "Lightweight Concrete Bag", description: "Amet ullam assumenda eligendi consectetur quae. Bl...", price: 72081, status: 2, published_date: "2016-07-16 00:00:00", created_at: "2016-07-17 05:15:07", updated_at: "2016-07-17 05:15:07", seller_id: 2>, #<Item id: 13, title: "Mediocre Plastic Computer", description: "Excepturi modi est non qui iusto. Molestiae offici...", price: 94357, status: 2, published_date: "2016-07-15 00:00:00", created_at: "2016-07-17 05:15:07", updated_at: "2016-07-17 05:15:07", seller_id: 2>, #<Item id: 15, title: "Incredible Plastic Bag", description: "Vel voluptas ducimus soluta atque voluptatem eum. ...", price: 15661, status: 2, published_date: "2016-07-14 00:00:00", created_at: "2016-07-17 05:15:07", updated_at: "2016-07-17 05:15:07", seller_id: 2>, #<Item id: 16, title: "Lightweight Iron Watch", description: "Id sequi rerum dolor sit sunt nemo laborum. Omnis ...", price: 65306, status: 4, published_date: "2016-07-11 00:00:00", created_at: "2016-07-17 05:15:07", updated_at: "2016-07-17 05:15:07", seller_id: 1>, #<Item id: 17, title: "Rustic Linen Chair", description: "Explicabo qui ad nihil. Voluptatem placeat autem. ...", price: 39752, status: 4, published_date: "2016-07-04 00:00:00", created_at: "2016-07-17 05:15:07", updated_at: "2016-07-17 05:15:07", seller_id: 1>, #<Item id: 18, title: "Mediocre Copper Car", description: "Minus qui ut est non vero saepe. Qui sed quos et v...", price: 87765, status: 4, published_date: "2016-07-05 00:00:00", created_at: "2016-07-17 05:15:07", updated_at: "2016-07-17 05:15:07", seller_id: 1>]>

回答1:

I think the approach for fetching associated objects with has_many through is to do something like:

Item.joins(:categories).where(categories: {id: 1})

I'm yet to find a better approach of doing this.

来源：https://stackoverflow.com/questions/38423774/rails-4-has-many-through-association-finding-associated-objects

标签

ruby-on-rails

activerecord

associations