DEV: Move hotlinked image information into a dedicated table (#16585)

This will make future changes to the 'pull hotlinked images' system easier. This commit should not introduce any functional change.

For now, the old post_custom_field data is kept in the database. This will be dropped in a future commit.

db/migrate/20220428094026_create_post_hotlinked_media.rb

@@ -0,0 +1,75 @@
+# frozen_string_literal: true
+
+class CreatePostHotlinkedMedia < ActiveRecord::Migration[6.1]
+  def change
+    reversible do |dir|
+      dir.up do
+        execute <<~SQL
+          CREATE TYPE hotlinked_media_status AS ENUM('downloaded', 'too_large', 'download_failed', 'upload_create_failed')
+        SQL
+      end
+      dir.down do
+        execute <<~SQL
+          DROP TYPE hotlinked_media_status
+        SQL
+      end
+    end
+
+    create_table :post_hotlinked_media do |t|
+      t.bigint :post_id, null: false
+      t.string :url, null: false
+      t.column :status, :hotlinked_media_status, null: false
+      t.bigint :upload_id
+      t.timestamps
+
+      t.index [:post_id, :url], unique: true
+    end
+
+    reversible do |dir|
+      dir.up do
+        execute <<~SQL
+          INSERT INTO post_hotlinked_media (post_id, url, status, upload_id, created_at, updated_at)
+          SELECT
+            post_id,
+            obj.key AS url,
+            'downloaded',
+            obj.value::bigint AS upload_id,
+            pcf.created_at,
+            pcf.updated_at
+          FROM post_custom_fields pcf
+          JOIN json_each_text(pcf.value::json) obj ON true
+          JOIN uploads ON obj.value::bigint = uploads.id
+          WHERE name='downloaded_images'
+        SQL
+
+        execute <<~SQL
+          INSERT INTO post_hotlinked_media (post_id, url, status, upload_id, created_at, updated_at)
+          SELECT
+            post_id,
+            url.value,
+            'download_failed',
+            NULL,
+            pcf.created_at,
+            pcf.updated_at
+          FROM post_custom_fields pcf
+          JOIN json_array_elements_text(pcf.value::json) url ON true
+          WHERE name='broken_images'
+        SQL
+
+        execute <<~SQL
+          INSERT INTO post_hotlinked_media (post_id, url, status, upload_id, created_at, updated_at)
+          SELECT
+            post_id,
+            url.value,
+            'too_large',
+            NULL,
+            pcf.created_at,
+            pcf.updated_at
+          FROM post_custom_fields pcf
+          JOIN json_array_elements_text(pcf.value::json) url ON true
+          WHERE name='large_images'
+        SQL
+      end
+    end
+  end
+end