FIX: Switch out topic template if the user hasn't changed it

app/assets/javascripts/discourse/models/composer.js.es6

@@ -3,6 +3,7 @@ import Topic from 'discourse/models/topic';
 import { throwAjaxError } from 'discourse/lib/ajax-error';
 import Quote from 'discourse/lib/quote';
 import Draft from 'discourse/models/draft';
+import computed from 'ember-addons/ember-computed-decorators';
 
 const CLOSED = 'closed',
       SAVING = 'saving',
@@ -35,11 +36,32 @@ const CLOSED = 'closed',
       };
 
 const Composer = RestModel.extend({
+  _categoryId: null,
 
   archetypes: function() {
     return this.site.get('archetypes');
   }.property(),
 
+
+  @computed
+  categoryId: {
+    get() { return this._categoryId; },
+
+    // We wrap categoryId this way so we can fire `applyTopicTemplate` with
+    // the previous value as well as the new value
+    set(categoryId) {
+      const oldCategoryId = this._categoryId;
+
+      if (Ember.isEmpty(categoryId)) { categoryId = null; }
+      this._categoryId = categoryId;
+
+      if (oldCategoryId !== categoryId) {
+        this.applyTopicTemplate(oldCategoryId, categoryId);
+      }
+      return categoryId;
+    }
+  },
+
   creatingTopic: Em.computed.equal('action', CREATE_TOPIC),
   creatingPrivateMessage: Em.computed.equal('action', PRIVATE_MESSAGE),
   notCreatingPrivateMessage: Em.computed.not('creatingPrivateMessage'),
@@ -56,6 +78,7 @@ const Composer = RestModel.extend({
   viewOpen: Em.computed.equal('composeState', OPEN),
   viewDraft: Em.computed.equal('composeState', DRAFT),
 
+
   composeStateChanged: function() {
     var oldOpen = this.get('composerOpened');
 
@@ -339,20 +362,24 @@ const Composer = RestModel.extend({
     this.keyValueStore.set({ key: 'composer.showPreview', value: this.get('showPreview') });
   },
 
-  applyTopicTemplate: function() {
+  applyTopicTemplate(oldCategoryId, categoryId) {
     if (this.get('action') !== CREATE_TOPIC) { return; }
-    if (!Ember.isEmpty(this.get('reply'))) { return; }
+    let reply = this.get('reply');
 
-    const categoryId = this.get('categoryId');
+    // If the user didn't change the template, clear it
-    const category = this.site.categories.find((c) => c.get('id') === categoryId);
+    if (oldCategoryId) {
+      const oldCat = this.site.categories.findProperty('id', oldCategoryId);
+      if (oldCat && (oldCat.get('topic_template') === reply)) {
+        reply = "";
+      }
+    }
+
+    if (!Ember.isEmpty(reply)) { return; }
+    const category = this.site.categories.findProperty('id', categoryId);
     if (category) {
-      const topicTemplate = category.get('topic_template');
+      this.set('reply', category.get('topic_template') || "");
-      if (!Ember.isEmpty(topicTemplate)) {
-        this.set('reply', topicTemplate);
     }
-    }
+  },
-
-  }.observes('categoryId'),
 
   /*
      Open a composer
@@ -397,14 +424,15 @@ const Composer = RestModel.extend({
       }
     }
 
-    const categoryId = opts.categoryId || this.get('topic.category.id');
     this.setProperties({
-      categoryId,
       archetypeId: opts.archetypeId || this.site.get('default_archetype'),
       metaData: opts.metaData ? Em.Object.create(opts.metaData) : null,
       reply: opts.reply || this.get("reply") || ""
     });
 
+    // We set the category id separately for topic templates on opening of composer
+    this.set('categoryId', opts.categoryId || this.get('topic.category.id'));
+
     if (opts.postId) {
       this.set('loading', true);
       this.store.find('post', opts.postId).then(function(post) {