Added ability to fetch collection posts by slug

closes https://github.com/TryGhost/Arch/issues/27

- We need a more convenient method of fetching posts belonging to a collection than by collection's "id". This change adds an alias to the existing endpoint `GET /collections/:id/posts/`. A non-valid ObjectID in the parameter is treated as a slug.

ghost/collections/src/CollectionsService.ts

@@ -1,5 +1,6 @@
 import logging from '@tryghost/logging';
 import tpl from '@tryghost/tpl';
+import ObjectID from 'bson-objectid';
 import {Collection} from './Collection';
 import {CollectionResourceChangeEvent} from './CollectionResourceChangeEvent';
 import {CollectionRepository} from './CollectionRepository';
@@ -350,8 +351,19 @@ export class CollectionsService {
         };
     }
 
+    /**
+     * @param id {string | ObjectID} - collection id or slug
+     * @param pagingOpts {QueryOptions}
+     * @returns
+     */
     async getAllPosts(id: string, {limit = 15, page = 1}: QueryOptions): Promise<{data: CollectionPostListItemDTO[], meta: any}> {
-        const collection = await this.getById(id);
+        let collection;
+
+        if (ObjectID.isValid(id)) {
+            collection = await this.getById(id);
+        } else {
+            collection = await this.getBySlug(id);
+        }
 
         if (!collection) {
             throw new NotFoundError({

ghost/collections/test/collections.test.ts

@@ -130,7 +130,7 @@ describe('CollectionsService', function () {
     });
 
     describe('getAllPosts', function () {
-        it('Can get paged posts of a collection', async function () {
+        it('Can get paged posts of a collection by collection id', async function () {
             const collection = await collectionsService.createCollection({
                 title: 'testing paging',
                 type: 'manual'
@@ -161,6 +161,38 @@ describe('CollectionsService', function () {
             assert.equal(postsPage2.data[1].id, posts[3].id, 'Second post should be the correct one');
         });
 
+        it('Can get paged posts of a collection by collection slug', async function () {
+            const collection = await collectionsService.createCollection({
+                title: 'testing fetch by slug',
+                slug: 'testing-fetch-by-slug',
+                type: 'manual'
+            });
+
+            for (const post of posts) {
+                await collectionsService.addPostToCollection(collection.id, post);
+            }
+
+            const postsPage1 = await collectionsService.getAllPosts(collection.slug, {page: 1, limit: 2});
+
+            assert.ok(postsPage1, 'Posts should be returned');
+            assert.equal(postsPage1.meta.pagination.page, 1, 'Page should be 1');
+            assert.equal(postsPage1.meta.pagination.limit, 2, 'Limit should be 2');
+            assert.equal(postsPage1.meta.pagination.pages, 2, 'Pages should be 2');
+            assert.equal(postsPage1.data.length, 2, 'There should be 2 posts');
+            assert.equal(postsPage1.data[0].id, posts[0].id, 'First post should be the correct one');
+            assert.equal(postsPage1.data[1].id, posts[1].id, 'Second post should be the correct one');
+
+            const postsPage2 = await collectionsService.getAllPosts(collection.slug, {page: 2, limit: 2});
+
+            assert.ok(postsPage2, 'Posts should be returned');
+            assert.equal(postsPage2.meta.pagination.page, 2, 'Page should be 2');
+            assert.equal(postsPage2.meta.pagination.limit, 2, 'Limit should be 2');
+            assert.equal(postsPage2.meta.pagination.pages, 2, 'Pages should be 2');
+            assert.equal(postsPage2.data.length, 2, 'There should be 2 posts');
+            assert.equal(postsPage2.data[0].id, posts[2].id, 'First post should be the correct one');
+            assert.equal(postsPage2.data[1].id, posts[3].id, 'Second post should be the correct one');
+        });
+
         it('Throws when trying to get posts of a collection that does not exist', async function () {
             await assert.rejects(async () => {
                 await collectionsService.getAllPosts('fake id', {});

ghost/core/test/e2e-api/admin/collections.test.js

@@ -137,6 +137,19 @@ describe('Collections API', function () {
                     collection_posts: Array(2).fill(matchCollectionPost)
                 });
         });
+
+        it('Can browse Collections Posts using collection slug', async function () {
+            await agent
+                .get(`/collections/index/posts/`)
+                .expectStatus(200)
+                .matchHeaderSnapshot({
+                    'content-version': anyContentVersion,
+                    etag: anyEtag
+                })
+                .matchBodySnapshot({
+                    collection_posts: Array(11).fill(matchCollectionPost)
+                });
+        });
     });
 
     it('Can read a Collection', async function () {