Methods
W
Instance Public methods
with_scope(scope = {}, action = :merge, &block)

#with_scope lets you apply options to inner block incrementally. It takes a hash and the keys must be :find or :create. :find parameter is Relation while :create parameters are an attributes hash.

class Article < ActiveRecord::Base
  def self.create_with_scope
    with_scope(:find => where(:blog_id => 1), :create => { :blog_id => 1 }) do
      find(1) # => SELECT * from articles WHERE blog_id = 1 AND id = 1
      a = create(1)
      a.blog_id # => 1
    end
  end
end

In nested scopings, all previous parameters are overwritten by the innermost rule, with the exception of where, includes, and joins operations in Relation, which are merged.

joins operations are uniqued so multiple scopes can join in the same table without table aliasing problems. If you need to join multiple tables, but still want one of the tables to be uniqued, use the array of strings format for your joins.

class Article < ActiveRecord::Base
  def self.find_with_scope
    with_scope(:find => where(:blog_id => 1).limit(1), :create => { :blog_id => 1 }) do
      with_scope(:find => limit(10)) do
        all # => SELECT * from articles WHERE blog_id = 1 LIMIT 10
      end
      with_scope(:find => where(:author_id => 3)) do
        all # => SELECT * from articles WHERE blog_id = 1 AND author_id = 3 LIMIT 1
      end
    end
  end
end

You can ignore any previous scopings by using the with_exclusive_scope method.

class Article < ActiveRecord::Base
  def self.find_with_exclusive_scope
    with_scope(:find => where(:blog_id => 1).limit(1)) do
      with_exclusive_scope(:find => limit(10)) do
        all # => SELECT * from articles LIMIT 10
      end
    end
  end
end

Note: the :find scope also has effect on update and deletion methods, like update_all and delete_all.

Source: show | on GitHub 

# File activerecord/lib/active_record/scoping.rb, line 60
def with_scope(scope = {}, action = :merge, &block)
  # If another Active Record class has been passed in, get its current scope
  scope = scope.current_scope if !scope.is_a?(Relation) && scope.respond_to?(:current_scope)

  previous_scope = self.current_scope

  if scope.is_a?(Hash)
    # Dup first and second level of hash (method and params).
    scope = scope.dup
    scope.each do |method, params|
      scope[method] = params.dup unless params == true
    end

    scope.assert_valid_keys([ :find, :create ])
    relation = construct_finder_arel(scope[:find] || {})
    relation.default_scoped = true unless action == :overwrite

    if previous_scope && previous_scope.create_with_value && scope[:create]
      scope_for_create = if action == :merge
        previous_scope.create_with_value.merge(scope[:create])
      else
        scope[:create]
      end

      relation = relation.create_with(scope_for_create)
    else
      scope_for_create = scope[:create]
      scope_for_create ||= previous_scope.create_with_value if previous_scope
      relation = relation.create_with(scope_for_create) if scope_for_create
    end

    scope = relation
  end

  scope = previous_scope.merge(scope) if previous_scope && action == :merge

  self.current_scope = scope
  begin
    yield
  ensure
    self.current_scope = previous_scope
  end
end
Instance Protected methods
with_exclusive_scope(method_scoping = {}, &block)

Works like #with_scope, but discards any nested properties.

Source: show | on GitHub 

# File activerecord/lib/active_record/scoping.rb, line 107
      def with_exclusive_scope(method_scoping = {}, &block)
        if method_scoping.values.any? { |e| e.is_a?(ActiveRecord::Relation) }
          raise ArgumentError, "  New finder API can not be used with_exclusive_scope. You can either call unscoped to get an anonymous scope not bound to the default_scope:

  User.unscoped.where(:active => true)

  Or call unscoped with a block:

  User.unscoped do
  User.where(:active => true).all
  end

"
        end
        with_scope(method_scoping, :overwrite, &block)
      end