add migration file to delete wrong index

src/migrations/20191127023815-drop-wrong-index-of-page-tag-relation.js

@@ -0,0 +1,27 @@
+require('module-alias/register');
+const logger = require('@alias/logger')('growi:migrate:drop-wrong-index-of-page-tag-relation');
+
+const mongoose = require('mongoose');
+const config = require('@root/config/migrate');
+
+async function dropIndexIfExists(collection, indexName) {
+  if (await collection.indexExists(indexName)) {
+    await collection.dropIndex(indexName);
+  }
+}
+
+module.exports = {
+  async up(db) {
+    logger.info('Apply migration');
+    mongoose.connect(config.mongoUri, config.mongodb.options);
+
+    const collection = db.collection('pagetagrelations');
+    await dropIndexIfExists(collection, 'page_1_user_1');
+
+    logger.info('Migration has successfully applied');
+  },
+
+  down(db) {
+    // do not rollback
+  },
+};