Assigned existing email records and related posts to the default newsletter (#14695)

refs https://github.com/TryGhost/Team/issues/1571

- With the addition of multiple newsletters, all emails sent previously should be assigned to the default newsletter
- This will make sure that the sent count for the default newsletter displays correctly

core/server/data/migrations/versions/5.0/2022-05-04-15-24-map-existing-emails-to-default-newsletter.js

@@ -0,0 +1,44 @@
+const logging = require('@tryghost/logging');
+const {createTransactionalMigration} = require('../../utils');
+
+module.exports = createTransactionalMigration(
+    async function up(knex) {
+        logging.info('Linking existing emails and related posts to the default newsletter');
+
+        // Get the default newsletter
+        // Note we intentionally use the default newsletter slug instead of the usual orderBy logic
+        let newsletter = await knex('newsletters')
+            .where('slug', 'default-newsletter')
+            .first('id', 'slug');
+
+        if (!newsletter) {
+            // Fall back to orderBy - just in case
+            logging.warn(`Original default newsletter not found - using first in sort order`);
+            newsletter = await knex('newsletters')
+                .where('status', 'active')
+                .orderBy('sort_order', 'asc')
+                .orderBy('created_at', 'asc')
+                .orderBy('id', 'asc')
+                .first('id', 'slug');
+        }
+
+        if (!newsletter) {
+            logging.error(`Newsletter not found - skipping`);
+            return;
+        }
+
+        logging.info(`Assigning existing emails to newsletter ID ${newsletter.id} (${newsletter.slug})`);
+
+        // Set newsletter ID only on posts with related email records without a newsletter assigned
+        await knex('posts')
+            .update('newsletter_id', newsletter.id)
+            .whereIn('id', knex.raw('SELECT post_id FROM emails WHERE emails.newsletter_id IS NULL'));
+
+        await knex('emails')
+            .update('newsletter_id', newsletter.id)
+            .whereNull('newsletter_id');
+    },
+    async function down() {
+        // Not required
+    }
+);