DEV: Add polymorphic bookmarkable columns (#15454)

We are planning on attaching bookmarks to more and
more other models, so it makes sense to make a polymorphic
relationship to handle this. This commit adds the new
columns and backfills them in the bookmark table, and
makes sure that any new bookmark changes fill in the columns
via DB triggers.

This way we can gradually change the frontend and backend
to use these new columns, and eventually delete the
old post_id and for_topic columns in `bookmarks`.

app/models/bookmark.rb

@@ -4,6 +4,7 @@ class Bookmark < ActiveRecord::Base
   belongs_to :user
   belongs_to :post
   has_one :topic, through: :post
+  belongs_to :bookmarkable, polymorphic: true
 
   delegate :topic_id, to: :post
 
@@ -171,9 +172,12 @@ end
 #  auto_delete_preference :integer          default(0), not null
 #  pinned                 :boolean          default(FALSE)
 #  for_topic              :boolean          default(FALSE), not null
+#  bookmarkable_id        :integer
+#  bookmarkable_type      :string
 #
 # Indexes
 #
+#  idx_bookmarks_user_polymorphic_unique                 (user_id,bookmarkable_type,bookmarkable_id) UNIQUE
 #  index_bookmarks_on_post_id                            (post_id)
 #  index_bookmarks_on_reminder_at                        (reminder_at)
 #  index_bookmarks_on_reminder_set_at                    (reminder_set_at)

app/models/post.rb

@@ -46,6 +46,9 @@ class Post < ActiveRecord::Base
   has_many :uploads, through: :post_uploads
 
   has_one :post_stat
+
+  # When we are ready we can add as: :bookmarkable here to use the
+  # polymorphic association.
   has_many :bookmarks
 
   has_one :incoming_email

app/models/topic.rb

@@ -203,7 +203,15 @@ class Topic < ActiveRecord::Base
   belongs_to :category
   has_many :category_users, through: :category
   has_many :posts
+
+  # When we are ready we can add as: :bookmarkable here to use the
+  # polymorphic association.
+  #
+  # At that time we may also want to make another association for example
+  # :topic_bookmarks that get all of the bookmarks for that topic's bookmarkable id
+  # and type, because this one gets all of the post bookmarks.
   has_many :bookmarks, through: :posts
+
   has_many :ordered_posts, -> { order(post_number: :asc) }, class_name: "Post"
   has_many :topic_allowed_users
   has_many :topic_allowed_groups