How to get random unread article? - sql

Question

Database table articles contains 10.000 rows (articles)
I want to get a random article, mark it as read and never get it again,
so my next get request should return random article except that one.
There are thousands of users like me and all of them are using this table.
How do I implement this? How do I mark those articles, and how do I search only unread articles?
I was trying to create the column relatedUsers in the Articles table, which is filled out with relations to user objects, who has read the article. I was using search query: relatedUsers.objectId!=currentUserId But when this column get 2+ relatedUsers, this solutions stops working, because the database returns articles, which has at least one relatedUser, who's objectId isn't equal to currentUserId (means all users).
I'm using Backendless.com right now, but if I guess, the solution should be applicable to any database, including backendless. If not, then what kind of database should I use for this?

Answer 1

What you can do now is:
Create a table named UserArticle with fields articleId and userId
When you add a user or an article, you should also add to this table relations to all articles or users respectively; this way your UserArticle table will contain the relations between articles and users who haven't yet read those articles
And in order to get a random not read article for user, you:
Retrieve items from UserArticle where userId = yourCurrentUserId
Randomly select one of them and retrieve an article by its ID
Remove the selected article from UserArticle by articleId and userId, meaning that the user has read the article
This approach is the most suitable in your case. It involves a little more than one request, but for now your requirements cannot be fit better.
Also we (the Backendless team) are working on a keyword like contains, using which you would be able to have only a relation to Users table and get not read articles in one request, so the suggested approach is pretty temporary.

Answer 2

Getting a random number depends on the database. But the basic idea is to have a table called UserArticles with one row per user and per article already read (and perhaps other information such as the time).
Then, you can do:
select a.*
from articles a
where not exists (select 1
from userarticles ua
where a.article_id = ua.article_id and ua.user_id = $user_id
)
order by rand()
limit 1;
The order by rand() limit 1 is definitely database-dependent. But, it gives you the idea of how to approach the problem.
Once you have selected the article, then you insert this information into UserArticles:
insert into UserArticles(user_id, article_id)
values ($user_id, $article_id);
where $article_id refers to the article retrieved in the previous step.
Note: this should be fine with respect to race conditions, unless a single user can have multiple simultaneous connections to the database asking for the same information. Handling that case requires more knowledge about the database being used.