Postgresql select based on another select with order, limit and agregate - postgresql

Question

I have a tables like:
books:
id | title | rating_avr | visible
1 | 'Overlord' | 5 | true
2 | 'Avengers' | 10 | false
tags_books:
tag_id | book_id | likes
1 | 1 | 5
2 | 1 | 25
1 | 2 | 11
tags:
id | name
1 | 'Adventure'
2 | 'Drama'
Now i need to load books that have tag 'Drama' with LIMIT, ORDER and agregate tags for each book.
I managed to achive this using query:
SELECT b.id, b.title, b.rating_avr, json_agg(json_build_object('id', tb2.tag_id)) as tags
FROM books b
LEFT JOIN tags_books tb ON tb.book_id = b.id
LEFT JOIN tags_books tb2 ON tb2.book_id = b.id
WHERE tb.tag_id = 1 AND b.visible=true
GROUP BY b.id ORDER BY b.rating_avr DESC LIMIT 5
What i'm curious about:
1) Is it ok to join same table 2 times? First is for where clause and second to agregate tags.
2) How can i order agregated tags based on likes?
3) Is it a right approach, or maybe there is better way to do it?

Answer 1

It is strange that in your query, you don't use the table tags, although you want to fetch books with tag 'Drama' which is a column in the table tags.
What I would do is first get the ids of all the books with tag 'Drama' with a query like this:
SELECT b.id FROM books b
INNER JOIN tags_books tb ON tb.book_id = b.id
INNER JOIN tags t ON t.id = tb.tag_id
WHERE t.name = 'Drama' AND b.visible=true
and then use it to get the result:
SELECT
b.id, b.title, b.rating_avr,
json_agg(json_build_object('id', tb.tag_id) order by tb.likes desc) as tags
FROM books b INNER JOIN tags_books tb
ON tb.book_id = b.id
WHERE b.id IN (
SELECT b.id FROM books b
INNER JOIN tags_books tb ON tb.book_id = b.id
INNER JOIN tags t ON t.id = tb.tag_id
WHERE t.name = 'Drama' AND b.visible=true
)
GROUP BY b.id, b.title, b.rating_avr
ORDER BY b.rating_avr DESC LIMIT 5
See the demo.
Results:
> id | title | rating_avr | tags
> -: | :------- | ---------: | :-----------------------
> 1 | Overlord | 5 | [{"id" : 2}, {"id" : 1}]