Ruby on Rails - beware of default scope in ruby on rails - ruby on rails tutorial - rails guides - rails tutorial - ruby rails
ActiveRecord includes default_scope, to automatically scope a model by default.
class Post
default_scope ->{ where(published: true).order(created_at: :desc) }
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- Above code will serve posts which are already published when we perform any query on the model.
Post.all # will only list published posts Clicking "Copy Code" button will copy the code into the clipboard - memory. Please paste(Ctrl+V) it in your destination. The code will get pasted. Happy coding from Wikitechy - ruby on rails tutorial - rails guides - ruby rails - rubyonrails - learn ruby on rails - team
- That scope, while innocuous-looking, has multiple hidden side-effect that we may not want.
- default_scope and order
- Since we declared an order in the default_scope, calling order on Post will be added as additional orders instead of overriding the default.
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SELECT "posts".* FROM "posts" WHERE "posts"."published" = 't' ORDER BY "posts"."created_at" DESC, "posts"."updated_at" DESCClicking "Copy Code" button will copy the code into the clipboard - memory. Please paste(Ctrl+V) it in your destination. The code will get pasted. Happy coding from Wikitechy - ruby on rails tutorial - rails guides - ruby rails - rubyonrails - learn ruby on rails - team
default_scope and model initialization
- With any other ActiveRecord::Relation, default_scope will alter the default state of models initialized from it.
- Above example, Post has where(published: true) set by default, and so new models from Post will also have it set.
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unscoped
- default_scope can nominally be cleared by calling unscoped first, but this also has side-effects.
Example
class Post < Document
default_scope ->{ where(published: true).order(created_at: :desc) }
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Default, queries against Post will be scoped to type columns containing 'Post". But unscoped will clear this along with our own default_scope, so if we use unscoped we have to remember to account for this as well.
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unscoped and Model Associations
Assume a relationship between Post and User
class Post < ApplicationRecord
belongs_to :user
default_scope ->{ where(published: true).order(created_at: :desc) }
end
class User < ApplicationRecord
has_many :posts
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- Getting an individual User.
- We can see the posts related to it:
user = User.find(1)
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SELECT "posts".* FROM "posts" WHERE "posts"."published" = 't' AND "posts"."user_id" = ? ORDER BY "posts"."created_at" DESC [["user_id", 1]]Clicking "Copy Code" button will copy the code into the clipboard - memory. Please paste(Ctrl+V) it in your destination. The code will get pasted. Happy coding from Wikitechy - ruby on rails tutorial - rails guides - ruby rails - rubyonrails - learn ruby on rails - team
- But we want to clear the default_scope from the posts relation, so we use unscoped.
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SELECT "posts".* FROM "posts"Clicking "Copy Code" button will copy the code into the clipboard - memory. Please paste(Ctrl+V) it in your destination. The code will get pasted. Happy coding from Wikitechy - ruby on rails tutorial - rails guides - ruby rails - rubyonrails - learn ruby on rails - team
This wipes out the user_id condition as well as the default_scope.
Example
- There are situations where using default_scope is justifiable.
- Assume a multi-tenant system where multiple subdomains are served from the same application but with isolated data. One way to achieve this isolation is through default_scope.
- The downsides in other cases become upsides here.
class ApplicationRecord < ActiveRecord::Base
def self.inherited(subclass)
super
return unless subclass.superclass == self
return unless subclass.column_names.include? 'tenant_id'
subclass.class_eval do
default_scope ->{ where(tenant_id: Tenant.current_id) }
end
end
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- All we need to do is set Tenant.current_id to something early in the request, and any table that contains tenant_id will automatically become scoped without any additional code. Instantiating records will automatically inherit the tenant id they were created under.
- The important thing about this use-case is that the scope is set once per request, and it doesn't change. The only cases we will need unscoped here are special cases like background workers that run outside of a request scope.