Add Mark For Destruction as a rails til

rails/mark_for_destruction.md

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+# Mark For Destruction
+
+Do you have some complicated logic or criteria for deleting associated
+records? [ActiveRecord's
+`#mark_for_destruction`](http://api.rubyonrails.org/classes/ActiveRecord/AutosaveAssociation.html#method-i-mark_for_destruction) may come in handy.
+
+Let's say we have _users_ who author _articles_. We want to delete some of
+the user's articles based on some criteria -- those articles that have odd
+`id`s.
+
+```ruby
+> user = User.first
+#=> #<User...>
+> user.articles.each { |a| a.mark_for_destruction if a.id.odd? }
+#=> [#<Article...>, ...]
+> user.articles.find(1).marked_for_destruction?
+#=> true
+> user.articles.find(2).marked_for_destruction?
+#=> false
+```
+
+We've marked our articles for destruction and confirmed as much with the
+[`#marked_for_destruction?`](http://api.rubyonrails.org/classes/ActiveRecord/AutosaveAssociation.html#method-i-marked_for_destruction-3F) method. Now, to go through with the destruction, we just have to save the parent record -- the user.
+
+```ruby
+> user.save
+   (0.2ms)  BEGIN
+  User Exists (0.8ms)  SELECT  1 AS one FROM "users" WHERE ("users"."email" = 'person1@example.com' AND "users"."id" != 1) LIMIT 1
+  SQL (3.0ms)  DELETE FROM "articles" WHERE "articles"."id" = $1  [["id", 1]]
+  SQL (0.2ms)  DELETE FROM "articles" WHERE "articles"."id" = $1  [["id", 3]]
+   (2.1ms)  COMMIT
+=> true
+```
+
+Note: the parent record must have `autosave: true` declared on the
+association.
+
+```ruby
+class User < ActiveRecord::Base
+  has_many :articles, autosave: true
+end
+```