[Rails][Model] default_scope / scope

14 Scopes
Scoping allows you to specify commonly-used queries which can be referenced as method calls on the association objects or models. 

class User < ActiveRecord::Base
  scope :active, -> { where(status: 'active') }
end

class User < ActiveRecord::Base
  def self.active
    where(status: 'active')
  end
end

どちらの定義も同様のもの。
scopeはおおよそクラスメソッドのように利用する。

User.active
# => SELECT "users".* FROM "users" WHERE "users"."state" = 'active'

One important caveat is that default_scope will be overridden by scope and where conditions.

class User < ActiveRecord::Base
  default_scope  { where state: 'pending' }
  scope :active, -> { where state: 'active' }
  scope :inactive, -> { where state: 'inactive' }
end

User.all
# => SELECT "users".* FROM "users" WHERE "users"."state" = 'pending'

User.active
# => SELECT "users".* FROM "users" WHERE "users"."state" = 'active'

User.where(state: 'inactive')
# => SELECT "users".* FROM "users" WHERE "users"."state" = 'inactive'

As you can see above the default_scope is being overridden by both scope and where conditions.

default_scopeはscopeにも、whereによっても無効になる。

14.3 Applying a default scope
If we wish for a scope to be applied across all queries to the model we can use the default_scope method within the model itself.

class Client < ActiveRecord::Base
  default_scope { where("removed_at IS NULL") }
end

When queries are executed on this model, the SQL query will now look something like this:

SELECT * FROM clients WHERE removed_at IS NULL
If you need to do more complex things with a default scope, you can alternatively define it as a class method:

class Client < ActiveRecord::Base
  def self.default_scope
    # Should return an ActiveRecord::Relation.
  end
end

モデルのすべてのクエリに対してscopeを適用したいときはdefault_scopeを定義する。
self.default_scopeでも同様の効果。