添加站内消息，修复讨论节点丢失的问题

hydro/model/message.js

@@ -0,0 +1,72 @@
+const { MessageNotFoundError } = require('../error');
+const db = require('../service/db.js');
+
+const coll = db.collection('message');
+
+async function add(from, to, content) {
+    const res = await coll.insertOne({
+        from,
+        to,
+        fromUnread: false,
+        toUnread: true,
+        reply: [
+            {
+                from, unread: true, content, at: new Date(),
+            },
+        ],
+    });
+    return await coll.findOne({ _id: res.insertedId }); // eslint-disable-line no-return-await
+}
+
+async function get(_id) {
+    const doc = await coll.findOne({ _id });
+    if (!doc) throw new MessageNotFoundError();
+    return doc;
+}
+
+function getMany(query, sort, page, limit) {
+    return coll.find(query).sort(sort)
+        .skip((page - 1) * limit).limit(limit)
+        .toArray();
+}
+
+function del(_id) {
+    return coll.deleteOne({ _id });
+}
+
+function count(query) {
+    return coll.find(query).count();
+}
+
+function getMulti(uid) {
+    return coll.find({ $or: [{ from: uid }, { to: uid }] });
+}
+
+async function addReply(_id, from, content) {
+    const reply = {
+        from,
+        content,
+        unread: true,
+        at: new Date(),
+    };
+    await coll.updateOne({ _id }, { $push: { reply } });
+    return await coll.findOne({ _id }); // eslint-disable-line no-return-await
+}
+
+function index() {
+    return Promise.all([
+        coll.createIndex({ to: 1, _id: -1 }),
+        coll.createIndex({ from: 1, _id: -1 }),
+    ]);
+}
+
+global.Hydro.model.message = module.exports = {
+    count,
+    add,
+    get,
+    del,
+    getMany,
+    getMulti,
+    addReply,
+    index,
+};