Join for has_many_through in rails - ruby-on-rails

Question

I have 3 models as following.
class Order
belongs_to :item
belongs_to :category
end
class Item
has_many :orders
belongs_to :category
end
class Category
has_many :items
has_many :orders, through: :items
end
I want to join the tables like Order.joins(:item).joins(:category), but it's not working.
Desired SQL is
SELECT * FROM `orders`
INNER JOIN `items` ON `items`.`id` = `orders`.`item_id`
INNER JOIN `categories` ON `items`.`category_id` = `categories`.`id`
I hope your helps.

Answer 1

I'm a little confused because Order and Item both belongs_to Category and Category already has_many Orders with that setup, the :through option is unnecesary.
For your desired output I guess you want to do a nested join (order > item > category) instead of multiple joins (order > item+category)
https://guides.rubyonrails.org/active_record_querying.html#joining-multiple-associations
12.1.3.1 Joining Nested Associations (Single Level)
Article.joins(comments: :guest)
This produces:
SELECT articles.* FROM articles
INNER JOIN comments ON comments.article_id = articles.id
INNER JOIN guests ON guests.comment_id = comments.id
So, you should do something like Order.joins(item: :category)

Answer 2

The syntax you're looking for is
Order.joins(item: :category)
Check here for more information.

Answer 3

The proper way to setup these associations is:
class Order < ApplicationRecord
belongs_to :item
# This references items.category_id
has_one :category, through: :item
end
class Item < ApplicationRecord
has_many :orders
belongs_to :category
end
class Category < ApplicationRecord
has_many :item, through: :orders
has_many :orders
end
You want to remove the orders.category_id column (if it exists) and use an indirect association through the items table to avoid duplication. The semantics of belongs_to and has_one can be confusing but belongs_to assumes that the foreign key is on this model (orders), while has_one places it on the other models table (items).
This will let you join/include/eager_load the association with:
irb(main):002:0> Order.joins(:category)
Order Load (1.4ms) SELECT "orders".* FROM "orders" INNER JOIN "items" ON "items"."id" = "orders"."item_id" INNER JOIN "categories" ON "categories"."id" = "items"."category_id" LIMIT $1 [["LIMIT", 11]]
And as you can see Rails will handle joining the join table (items) automatically.
If you want both associations to be loaded you can use a hash or just list both:
Order.eager_load(item: :category)
Order.eager_load(:item, :category)