Ways to design a digest email implementation? - database

Question

I'm looking for some thoughts about how to design a digest email feature. I'm not concerned about the actual business code; instead I'd like to focus on the gist of it.
Let's tackle this with a known example: articles. Here's a general overview of some important features:
The user is able to choose the digest frequency (e.g. daily or weekly);
The digest only contains new articles;
"New articles" are to be considered relative to the previous digest that was sent to a specific user;
I've been thinking about the following:
Introduce per-user tracking of articles previously included in a digest and filter those out?
Requires a new database table;
Could become expensive when the table contains millions of rows;
What to do in case of including multiple types of models in the digest? Multiple tracking tables? Polymorphic table? ...?
Use article creation dates to include articles between current date and the chosen digest frequency?
Uses current date and information already present in the database, so no new tables required;
What happens when a user changes from daily to weekly emails? He could receive the same article again in the weekly digest. Should this edge case be considered? If so, how to mitigate?
For some reason the creation date of an article is being updated to today, positively triggering the date comparison again. Should this edge case be considered? If so, how to mitigate?
Or can you think of other ways to implement this feature?
I'm eager to learn your insights.

Answer 1

You can make an additional table that will contain information about digest subscription by each user. This way gives the ability to make a database design cleaner and more universal because mailing is a separate logical module. Aside from that, the additional table gives the ability to expand stored data about digest subscription easy in future. For example:
With help usage of this table, you would manage data easy. For example, you can select all recipient of daily digest:
SELECT *
FROM digest_subscription
WHERE interval_type = 'daily'
AND last_date_distribution <= NOW()
or select all recipient of the weekly digest
SELECT *
FROM digest_subscription
WHERE interval_type = 'weekly'
AND last_date_distribution <= NOW() - INTERVAL 7 DAY
Condition by interval type and compare the last date distribution by rule "equal or less" give the ability to avoid problems of untimely sending of emails (for example technical failures on a server, etc.)
Also, you can make correct articles list with help information of the last data distribution. Usage of the last data distribution gives the ability to avoid problems of interval change. For example:
SELECT *
FROM articles
WHERE created_at >= <the last date distribution of the user>
Of course, you don't avoid the problems of updated creation date. But you should minimize the reasons for that happening. For example, your code can update the modification date but your code shouldn't modify the creation date.