Rails - eager load has_many :through with condition on association - sql

Question

Let's say I have an Item model and Category model with has_many :through association:
class Item < ActiveRecord::Base
has_many :category_items
has_many :categories, through: category_items
end
class Category < ActiveRecord::Base
has_many :category_items
has_many :items, through: category_items
end
class CategoryItems < ActiveRecord::Base
belongs_to :category
belongs_to :items
end
now, I want to have a scope on items that will get all items that are in specific status (assume it has status attribute) for specific category. for example: get all items with status "in stock" and which belongs to category with id = 3, something like:
scope :in_stock_for_category, ->(category) { where(status: SOME_ENUMERATED_VALUE) ....
i'm missing the last part of the query to limit the result set to the specific category.
Thanks!

Answer 1

Since you don't have a category_id column in your items table, you need to join either category_items or cateogeries in your scope before you can specify a particular category's ID condition.
class Item < ActiveRecord::Base
scope :in_stock_for_category, -> do |category|
joins(:category_items).
where(category_items: {category_id: category.id}).
where(items: {status: SOME_ENUMERATED_VALUE}).
group("items.id") # grouping might be unnecessary since you're adding the where condition for the category's id
end
end
That will work. Or if you want to join categories, do the following:
class Item < ActiveRecord::Base
scope :in_stock_for_category, -> do |category|
joins(:categories).
where(categories: {id: category.id}).
where(items: {status: SOME_ENUMERATED_VALUE}).
group("items.id") # grouping might be unnecessary since you're adding the where condition for the category's id
end
end
If you already have a category however, it might be useful to create a has_many relationship for a items that have a certain status. Something like the following:
class Category < ActiveRecord::Base
has_many :in_stock_items, -> do
where(items: {status: SOME_ENUMERATED_VALUE})
end, through: :category_items, source: :item
end
Also, if you have a status scope in Item (something like scope :in_stock, -> { where(status: SOME_ENUMERATED_VALUE) }), you can most likely change the above has_many relationship to the following:
class Category < ActiveRecord::Base
has_many :in_stock_items, -> do
merge(Item.in_stock) # http://apidock.com/rails/ActiveRecord/SpawnMethods/merge
end, through: :category_items, source: :item
end
That should tidy things up.