Chore: Convert Message Sender to polymorphic (#740)

Fixes #680

db/migrate/20200418124534_add_sender_to_messages.rb

@@ -0,0 +1,25 @@
+class AddSenderToMessages < ActiveRecord::Migration[6.0]
+  def change
+    add_reference :messages, :sender, polymorphic: true, index: true
+    add_sender_from_message
+    remove_index :messages, name: 'index_messages_on_contact_id', column: 'contact_id'
+    remove_index :messages, name: 'index_messages_on_user_id', column: 'user_id'
+    remove_column :messages, :user_id, :integer # rubocop:disable Rails/BulkChangeTable
+    remove_column :messages, :contact_id, :integer
+  end
+
+  def add_sender_from_message
+    ::Message.find_in_batches do |messages_batch|
+      Rails.logger.info "migrated till #{messages_batch.first.id}\n"
+      messages_batch.each do |message|
+        # rubocop:disable Rails/SkipsModelValidations
+        message.update_columns(sender_id: message.user.id, sender_type: 'User') if message.user.present?
+        message.update_columns(sender_id: message.contact.id, sender_type: 'Contact') if message.contact.present?
+        if message.sender.nil?
+          message.update_columns(sender_id: message.conversation.contact.id, sender_type: 'Contact') if message.incoming?
+        end
+        # rubocop:enable Rails/SkipsModelValidations
+      end
+    end
+  end
+end