sir-ragna / flaskvotes

Very simple voting example

Tracking votes on content

Trying to track the number on votes on the content can be tricky. Here is a naive way to track votes and a proposed solution.

post_id	post_content	score
1	'hello'	3

When a user votes we increment or decrement the score counter based upon our user either upvoting(+1) or downvoting(-1).

Imagine our user Steve wants to cast a vote, how are we going to verify that Steve hasn't already voted? The solution to this is to track for which posts Steve already has voted.

Ideally however we do not separately track all the votes and the scores we are simply able to calculate the score by summing all the votes.

post_id	post_content
1	hello
2	Goodbye

Our voting table.

post_id	username	rating
1	Steve	1
1	Bert	-1
1	Rob	1
1	Olaf	1

Then when we need to retrieve the score we SUM the rating to get our total score.

SELECT post_id, SUM(rating) as 'score'
FROM votes
GROUP BY post_id

Retrieving the top 5 posts

Imagine we want to retrieve the top 5 posts. A naive way of doing this might be to write the following query.

SELECT posts.post_id, SUM(votes.rating)  as 'score'
FROM posts
JOIN votes on posts.post_id = votes.post_id
GROUP BY posts.post_id
ORDER BY score DESC
LIMIT 5

This however will not include posts that have not received any votes yet.

post_id	score
1	2

A JOIN is an INNER JOIN by default, this means you'll only get entries when something is found both in the votes table and in the posts table. Here the fix is to create a LEFT JOIN.

This will also return the posts that did not receive any rating.

post_id	score
1	2
2	NULL

This is already workable. Yet I prefer not having to deal with the NULL values in my Python code, hence I will ask to return 0 whenever NULL manifests. This can be achieved with the following code.

CASE WHEN votes.rating IS NULL THEN 0 ELSE votes.rating 

This is the equivalent of an if statement. Our full query as found in the get_top_posts-function.

SELECT posts.post_id, SUM(
	CASE WHEN votes.rating IS NULL 
		THEN 0 
		ELSE votes.rating 
	END) as 'score'
FROM posts
LEFT JOIN votes on posts.post_id = votes.post_id
GROUP BY posts.post_id
ORDER BY score DESC
LIMIT 5