UX: auto fix order when reordering categories (#6149)

* set correct position number when moving up/down

* UX: drop 'fix order' and auto re-order subcategory

- auto "fix position" on save
- place subcategories after parent category and maintain the relative
positions on save

test/javascripts/controllers/reorder-categories-test.js.es6

@@ -0,0 +1,50 @@
+import { mapRoutes } from "discourse/mapping-router";
+import createStore from "helpers/create-store";
+
+moduleFor("controller:reorder-categories", "controller:reorder-categories", {
+  beforeEach() {
+    this.registry.register("router:main", mapRoutes());
+  },
+  needs: ["controller:modal"]
+});
+
+QUnit.test("fixIndices set unique position number", function(assert) {
+  const store = createStore();
+
+  const categories = [];
+  for (let i = 0; i < 3; ++i) {
+    categories.push(store.createRecord("category", { id: i, position: 0 }));
+  }
+
+  const site = Ember.Object.create({ categories: categories });
+  const reorderCategoriesController = this.subject({ site });
+
+  reorderCategoriesController.fixIndices();
+
+  reorderCategoriesController.get("categoriesOrdered").forEach((category, index) => {
+    assert.equal(category.get("position"), index);
+  });
+});
+
+QUnit.test(
+  "fixIndices places subcategories after their parent categories, while maintaining the relative order",
+  function(assert) {
+    const store = createStore();
+
+    const parent = store.createRecord("category", { id: 1, position: 1, slug: "parent" });
+    const child1 = store.createRecord("category", { id: 2, position: 3, slug: "child1", parent_category_id: 1 });
+    const child2 = store.createRecord("category", { id: 3, position: 0, slug: "child2", parent_category_id: 1 });
+    const other = store.createRecord("category", { id: 4, position: 2, slug: "other" });
+
+    const categories = [child2, parent, other, child1];
+    const expectedOrderSlugs = ["parent", "child2", "child1", "other"];
+
+    const site = Ember.Object.create({ categories: categories });
+    const reorderCategoriesController = this.subject({ site });
+
+    reorderCategoriesController.fixIndices();
+
+    assert.deepEqual(reorderCategoriesController.get("categoriesOrdered").mapBy("slug"), expectedOrderSlugs);
+  }
+);
+