Allow multiple users to do !alert without overriding each other

src/data/Thread.js

@@ -169,8 +169,8 @@ class Thread {
     }
 
     if (this.alert_id) {
-      await this.setAlert(null);
-      await this.postSystemMessage(`<@!${this.alert_id}> New message from ${this.user_name}`);
+      await this.deleteAlerts();
+      await this.postSystemMessage(`${this.alert_id} New message from ${this.user_name}`);
     }
   }
 
@@ -449,14 +449,65 @@ class Thread {
    * @param {String} userId
    * @returns {Promise<void>}
    */
-  async setAlert(userId) {
+  async addAlert(userId) {
+    let alerts = await knex('threads')
+      .where('id', this.id)
+      .select('alert_id')
+      .first();
+    alerts = alerts.alert_id;
+
+    if (alerts == null) {
+      alerts = `<@!${userId}>`;
+    } else {
+      if (! alerts.includes(`<@!${userId}>`)) {
+        alerts += ` <@!${userId}>`;
+      }
+    }
+
     await knex('threads')
       .where('id', this.id)
       .update({
-        alert_id: userId
+        alert_id: alerts
       });
   }
 
+  /**
+   * @param {String} userId 
+   * @returns {Promise<void>}
+   */
+  async removeAlert(userId) {
+    let alerts = await knex('threads')
+      .where('id', this.id)
+      .select('alert_id')
+      .first();
+    alerts = alerts.alert_id;
+
+    if (! (alerts == null)) {
+      if (alerts.startsWith(`<@!${userId}>`)) { // we do this to properly handle the spacing between @s
+        alerts = alerts.replace(`<@!${userId}> `, "");
+      } else {
+        alerts = alerts.replace(` <@!${userId}>`, "");
+      }
+    }
+
+    await knex('threads')
+      .where('id', this.id)
+      .update({
+        alert_id: alerts
+      });
+  }
+
+  /**
+   * @returns {Promise<void>}
+   */
+  async deleteAlerts() {
+    await knex('threads')
+      .where('id', this.id)
+      .update({
+        alert_id: null
+      })
+  }
+
   /**
    * @returns {Promise<String>}
    */

src/modules/alert.js

@@ -1,10 +1,10 @@
 module.exports = ({ bot, knex, config, commands }) => {
   commands.addInboxThreadCommand('alert', '[opt:string]', async (msg, args, thread) => {
     if (args.opt && args.opt.startsWith('c')) {
-      await thread.setAlert(null);
-      await thread.postSystemMessage(`Cancelled new message alert`);
+      await thread.removeAlert(msg.author.id)
+      await thread.postSystemMessage(`Cancelled your new message alert`);
     } else {
-      await thread.setAlert(msg.author.id);
+      await thread.addAlert(msg.author.id);
       await thread.postSystemMessage(`Pinging ${msg.author.username}#${msg.author.discriminator} when this thread gets a new reply`);
     }
   });