Added newsletter param to email preview endpoint (#14820)

refs https://github.com/TryGhost/Team/issues/1621

We want to allow previewing emails based on the selected newsletter. The post model doesn't get a newsletter attached until a publish occurs so we can't use `post.newsletter` and need to give the option of specifying which newsletter to preview via query params.

- added support for `newsletter` query param on the `GET /email_previews/posts/:id/` endpoint where the value is a newsletter slug
- updated `generateEmailContent()` signature to use an options object because the order of memberSegment/newsletter arguments doesn't matter and is difficult to reason about if not named
- adjusted `generateEmailContent()` to fetch the newsletter matching the provided slug, falling back to the default newsletter if no slug is provided

core/server/api/canary/email-previews.js

@@ -15,7 +15,8 @@ module.exports = {
     read: {
         options: [
             'fields',
-            'memberSegment'
+            'memberSegment',
+            'newsletter'
         ],
         validation: {
             options: {
@@ -39,7 +40,10 @@ module.exports = {
                 });
             }
 
-            return emailPreview.generateEmailContent(model, frame.options.memberSegment);
+            return emailPreview.generateEmailContent(model, {
+                newsletter: frame.options.newsletter,
+                memberSegment: frame.options.memberSegment
+            });
         }
     },
     sendTestEmail: {

core/server/services/mega/email-preview.js

@@ -4,16 +4,22 @@ const models = require('../../models');
 class EmailPreview {
     /**
      * @param {Object} post - Post model object instance
-     * @param {String} memberSegment - member segment filter
+     * @param {Object} options
+     * @param {String} options.newsletter - newsletter slug
+     * @param {String} options.memberSegment - member segment filter
      * @returns {Promise<Object>}
      */
-    async generateEmailContent(post, memberSegment) {
-        let newsletter = post.relations.newsletter ?? await post.related('newsletter').fetch();
-        if (!newsletter) {
-            newsletter = await models.Newsletter.getDefaultNewsletter();
+    async generateEmailContent(post, {newsletter, memberSegment} = {}) {
+        let newsletterModel = post.relations.newsletter ?? await post.related('newsletter').fetch();
+        if (!newsletterModel) {
+            if (newsletter) {
+                newsletterModel = await models.Newsletter.findOne({slug: newsletter});
+            } else {
+                newsletterModel = await models.Newsletter.getDefaultNewsletter();
+            }
         }
 
-        let emailContent = await postEmailSerializer.serialize(post, newsletter, {
+        let emailContent = await postEmailSerializer.serialize(post, newsletterModel, {
             isBrowserPreview: true
         });
 

test/e2e-api/admin/email-previews.test.js

@@ -164,6 +164,42 @@ describe('Email Preview API', function () {
             const [preview] = jsonResponse.email_previews;
             preview.html.should.containEql(testUtils.DataGenerator.Content.newsletters[0].name);
         });
+
+        it('uses the newsletter provided through ?newsletter=slug', async function () {
+            const defaultNewsletter = await models.Newsletter.getDefaultNewsletter();
+            const selectedNewsletter = testUtils.DataGenerator.Content.newsletters[0];
+
+            selectedNewsletter.id.should.not.eql(defaultNewsletter.id, 'Should use a non-default newsletter for this test');
+
+            const post = testUtils.DataGenerator.forKnex.createPost({
+                id: ObjectId().toHexString(),
+                title: 'Post with email-only card',
+                slug: 'email-only-card',
+                mobiledoc: '{"version":"0.3.1","atoms":[],"cards":[],"markups":[["a",["href","https://ghost.org"]]],"sections":[[1,"p",[[0,[],0,"Testing "],[0,[0],1,"links"],[0,[],0," in email excerpt and apostrophes \'"]]]]}',
+                html: '<p>This is the actual post content...</p>',
+                plaintext: 'This is the actual post content...',
+                status: 'draft',
+                uuid: 'd52c42ae-2755-455c-80ec-70b2ec55c904'
+            });
+
+            await models.Post.add(post, {context: {internal: true}});
+
+            const res = await request
+                .get(localUtils.API.getApiQuery(`email_previews/posts/${post.id}/?newsletter=${selectedNewsletter.slug}`))
+                .set('Origin', config.get('url'))
+                .set('Accept', 'application/json')
+                .expect('Content-Type', /json/)
+                .expect('Cache-Control', testUtils.cacheRules.private)
+                .expect(200);
+
+            should.not.exist(res.headers['x-cache-invalidate']);
+            const jsonResponse = res.body;
+            should.exist(jsonResponse);
+            should.exist(jsonResponse.email_previews);
+
+            const [preview] = jsonResponse.email_previews;
+            preview.html.should.containEql(testUtils.DataGenerator.Content.newsletters[0].name);
+        });
     });
 
     describe('As Owner', function () {