topic fixup

app/assets/javascripts/discourse/app/models/topic.js

@@ -225,8 +225,14 @@ const Topic = RestModel.extend({
   },
 
   @discourseComputed("category_id")
-  category(categoryId) {
-    return Category.findById(categoryId);
+  category: {
+    get(categoryId) {
+      return Category.findById(categoryId);
+    },
+    set(category) {
+      this.set("category_id", category.id);
+      return category;
+    },
   },
 
   categoryClass: fmt("category.fullSlug", "category-%@"),

app/assets/javascripts/discourse/tests/unit/models/topic-test.js

@@ -5,6 +5,7 @@ import Category from "discourse/models/category";
 import Topic from "discourse/models/topic";
 import User from "discourse/models/user";
 import { discourseModule } from "discourse/tests/helpers/qunit-helpers";
+import createStore from "discourse/tests/helpers/create-store";
 
 discourseModule("Unit | Model | topic", function () {
   test("defaults", function (assert) {
@@ -36,7 +37,9 @@ discourseModule("Unit | Model | topic", function () {
   });
 
   test("lastUnreadUrl", function (assert) {
-    const category = EmberObject.create({
+    const store = createStore();
+    const category = store.createRecord("category", {
+      id: 1,
       navigate_to_first_post_after_read: true,
     });
 
@@ -45,10 +48,9 @@ discourseModule("Unit | Model | topic", function () {
       highest_post_number: 10,
       last_read_post_number: 10,
       slug: "hello",
+      category_id: category.id,
     });
 
-    topic.set("category", category);
-
     assert.equal(topic.get("lastUnreadUrl"), "/t/hello/101/1");
   });