add migration file

src/migrations/20200901034314-update-mail-transmission-fix.js

@@ -0,0 +1,34 @@
+require('module-alias/register');
+const logger = require('@alias/logger')('growi:migrate:update-mail-transmission-fix');
+
+const mongoose = require('mongoose');
+const config = require('@root/config/migrate');
+
+const { getModelSafely } = require('@commons/util/mongoose-utils');
+
+module.exports = {
+  async up(db, client) {
+    logger.info('Apply migration');
+    mongoose.connect(config.mongoUri, config.mongodb.options);
+
+    const Config = getModelSafely('Config') || require('@server/models/config')();
+
+    const transmissionMethod = await Config.findOne({
+      ns: 'crowi',
+      key: 'mail:transmissionMethod',
+    });
+
+    if (transmissionMethod == null) {
+      return logger.info('No need to change.');
+    }
+
+    transmissionMethod.value = JSON.stringify(transmissionMethod.value);
+    await transmissionMethod.save();
+
+    logger.info('Migration has successfully applied');
+  },
+
+  async down(db, client) {
+    // do not rollback
+  },
+};