Added comments & fixed

packages/app/src/migrations/20210913153942-migrate-slack-app-integration-schema.js

@@ -20,7 +20,9 @@ defaultSupportedCommandsNameForSingleUse.forEach((commandName) => {
 module.exports = {
   async up(db) {
     logger.info('Apply migration');
-    if (mongoose.connection.readyState === 1) {
+    // connect only if disconnected
+    // see: https://mongoosejs.com/docs/api/connection.html#connection_Connection-readyState
+    if (mongoose.connection.readyState === 0) {
       await mongoose.connect(getMongoUri(), mongoOptions);
     }