Added backfill migration for members created events (#15294)

closes https://github.com/TryGhost/Team/issues/1836

- Uses the timestamps from the members table to determine the timestamps for the events
- Clears the table when downgrading to prevent having multiple rows for the same member

Co-authored-by: Fabien "egg" O'Carroll <fabien@allou.is>

ghost/core/core/server/data/migrations/versions/5.11/2022-08-23-13-41-backfill-members-created-events.js

@@ -0,0 +1,32 @@
+const ObjectID = require('bson-objectid').default;
+const logging = require('@tryghost/logging');
+
+const {createTransactionalMigration} = require('../../utils');
+
+module.exports = createTransactionalMigration(
+    async function up(knex) {
+        const members = await knex('members')
+            .select('id', 'created_at');
+
+        if (members.length === 0) {
+            logging.warn(`Skipping migration because no members found`);
+            return;
+        }
+
+        const toInsert = members.map((member) => {
+            return {
+                id: ObjectID().toHexString(),
+                member_id: member.id,
+                created_at: member.created_at,
+                source: 'member'
+            };
+        });
+
+        logging.info(`Inserting ${toInsert.length} members created events`);
+        await knex.batchInsert('members_created_events', toInsert);
+    },
+    async function down(knex) {
+        logging.info(`Clearing all members created events`);
+        await knex('members_created_events').del();
+    }
+);