Closes https://github.com/TryGhost/Team/issues/1508
Refs https://github.com/TryGhost/Ghost/pull/14468
- Maps existing subscribers to the default newsletter
A note on performance:
We loop over the rows in a potentially large table (members) but I've minimised the impact by limiting the columns we fetch. The alternative is a raw SQL query like the one below: the SQL version takes ~0.9s vs ~1.1s for the migration (my laptop, ~30k members). The disadvantage of the raw SQL implementation is the approximation of the ObjectID (instead of a legit bson ID) which isn't sequential and may impact index size/performance.
```sql
insert into members_newsletters (id, member_id, newsletter_id)
select
substr(replace(uuid(),'-',''),1,24),
id,
'62595dcbfad4e031d85d166f'
from members
where members.subscribed=true;
```
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