DEV: Make Bookmark#post_id column nullable (#16287)

As we are gradually moving to having a polymorphic
bookmarkable relationship on the Bookmark table,
we need to make the post_id column nullable to be
able to develop and test the new columns, and
for cutover/migration purposes later as well.

db/post_migrate/20220325064954_make_some_bookmark_columns_nullable.rb

@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+class MakeSomeBookmarkColumnsNullable < ActiveRecord::Migration[6.1]
+  def up
+    change_column_null :bookmarks, :post_id, true
+    execute "ALTER TABLE bookmarks ADD CONSTRAINT enforce_post_id_or_bookmarkable CHECK (
+      (post_id IS NOT NULL) OR (bookmarkable_id IS NOT NULL AND bookmarkable_type IS NOT NULL)
+    )"
+  end
+
+  def down
+    DB.exec("UPDATE bookmarks SET post_id = bookmarkable_id WHERE bookmarkable_type = 'Post'")
+    DB.exec("UPDATE bookmarks SET post_id = (SELECT id FROM posts WHERE topic_id = bookmarkable_id AND post_number = 1), for_topic = TRUE WHERE bookmarkable_type = 'Topic'")
+    change_column_null :bookmarks, :post_id, false
+    execute "ALTER TABLE bookmarks DROP CONSTRAINT enforce_post_id_or_bookmarkable"
+  end
+end