diff --git a/packages/hydrooj/src/model/message.ts b/packages/hydrooj/src/model/message.ts
index fc1009c20..bc20d3a38 100644
--- a/packages/hydrooj/src/model/message.ts
+++ b/packages/hydrooj/src/model/message.ts
@@ -7,13 +7,13 @@ import { PRIV } from './builtin';
 import * as system from './system';
 import user from './user';
 
-const coll = db.collection('message');
-
 class MessageModel {
     static FLAG_UNREAD = 1;
     static FLAG_ALERT = 2;
     static FLAG_RICHTEXT = 4;
 
+    static coll = db.collection('message');
+
     @ArgMethod
     static async send(
         from: number, to: number,
@@ -23,29 +23,29 @@ class MessageModel {
         const mdoc: MessageDoc = {
             _id, from, to, content, flag,
         };
-        await coll.insertOne(mdoc);
+        await MessageModel.coll.insertOne(mdoc);
         if (from !== to) bus.broadcast('user/message', to, mdoc);
         if (flag & MessageModel.FLAG_UNREAD) await user.inc(to, 'unreadMsg', 1);
         return mdoc;
     }
 
     static async get(_id: ObjectId) {
-        return await coll.findOne({ _id });
+        return await MessageModel.coll.findOne({ _id });
     }
 
     @ArgMethod
     static async getByUser(uid: number) {
-        return await coll.find({ $or: [{ from: uid }, { to: uid }] }).sort('_id', -1).limit(1000).toArray();
+        return await MessageModel.coll.find({ $or: [{ from: uid }, { to: uid }] }).sort('_id', -1).limit(1000).toArray();
     }
 
     static async getMany(query: Filter<MessageDoc>, sort: any, page: number, limit: number) {
-        return await coll.find(query).sort(sort)
+        return await MessageModel.coll.find(query).sort(sort)
             .skip((page - 1) * limit).limit(limit)
             .toArray();
     }
 
     static async setFlag(messageId: ObjectId, flag: number) {
-        const result = await coll.findOneAndUpdate(
+        const result = await MessageModel.coll.findOneAndUpdate(
             { _id: messageId },
             { $bit: { flag: { xor: flag } } },
             { returnDocument: 'after' },
@@ -54,16 +54,16 @@ class MessageModel {
     }
 
     static async del(_id: ObjectId) {
-        return await coll.deleteOne({ _id });
+        return await MessageModel.coll.deleteOne({ _id });
     }
 
     @ArgMethod
     static count(query: Filter<MessageDoc> = {}) {
-        return coll.countDocuments(query);
+        return MessageModel.coll.countDocuments(query);
     }
 
     static getMulti(uid: number) {
-        return coll.find({ $or: [{ from: uid }, { to: uid }] });
+        return MessageModel.coll.find({ $or: [{ from: uid }, { to: uid }] });
     }
 
     static async sendNotification(message: string, ...args: any[]) {
@@ -78,7 +78,7 @@ class MessageModel {
 
 export async function apply() {
     return db.ensureIndexes(
-        coll,
+        MessageModel.coll,
         { key: { to: 1, _id: -1 }, name: 'to' },
         { key: { from: 1, _id: -1 }, name: 'from' },
     );
diff --git a/packages/hydrooj/src/model/user.ts b/packages/hydrooj/src/model/user.ts
index f32d02096..2df4a0948 100644
--- a/packages/hydrooj/src/model/user.ts
+++ b/packages/hydrooj/src/model/user.ts
@@ -441,10 +441,12 @@ class UserModel {
     }
 
     static delGroup(domainId: string, name: string) {
+        deleteUserCache(domainId);
         return collGroup.deleteOne({ domainId, name });
     }
 
     static updateGroup(domainId: string, name: string, uids: number[]) {
+        deleteUserCache(domainId);
         return collGroup.updateOne({ domainId, name }, { $set: { uids } }, { upsert: true });
     }
 }