How to aggregate upvote and post tables together in postgres?

I’m looking for a way to aggregate posts and the upvotes related to that post together using PostgreSQL. I want to make it similar to reddit’s upvote systems, where upvotes come already fetched and don’t need to be loaded on client side.

I have made these simple tables.

upvotes.sql

CREATE TABLE upvotes (
    id BIGSERIAL NOT NULL PRIMARY KEY,
    post_id BIGINT NOT NULL REFERENCES posts(id)
    user_id BIGINT NOT NULL REFERENCES users(id)
);

users.sql

CREATE TABLE users (
    id BIGSERIAL NOT NULL PRIMARY KEY,
    name VARCHAR NOT NULL
);

posts.sql

CREATE TABLE posts (
    id BIGSERIAL NOT NULL PRIMARY KEY,
    content VARCHAR NOT NULL
)

Basically I want to make my response look something like this:

post.json

{
    id: 1234,
    content: "foo",
    upvotes: 1234
}

Is there a way for doing this with a single query? I feel like this is a simple question but don’t really know how to make it work…

Answer

Here it is.

select to_jsonb(t.*)
from
(
  select p.id, p.content, count(*) upvotes_count
  from posts p 
  inner join upvotes uv on p.id = uv.post_id 
  group by p.id, p.contents
) t
-- where id = 1234;

Please note that if posts.id is primary key then p.contents is redundant in the group by clause. We do not actually pay attention who are the users that upvoted the post but only how many they are. So table users is not used in the query.