How to pass data between nested callbacks? - node.js

Question

I'm a newbie at Node JS, and I'm using NodeJS (v. 8.7.0), sqlite3 and Express.
I have two tables in a SQLite database:
releases (id, title, image)
links (id, url)
Each "release" has one or more "links" associated with it.
I can get all the releases using:
dbh.all("SELECT * FROM releases ORDER BY id DESC", (err, rows) => { ... })
And I can get all the links for a given release using:
dbh.all("SELECT * FROM links WHERE id = ?", (err, rows) => { ... })
But I can't figure out how to add a "links" property to each "release", which contains their corresponding links, so that I can feed the resulting object to Mustache, and generate a HTML page.
I know that storing hierarchical data inside of a relational database is not the best idea, and I could easily do this using PHP, but I really want to learn how to use NodeJS.
This is what I've come up so far:
var sqlite3 = require("sqlite3")
function main() {
db = new sqlite3.Database("releases.sqlite3")
all = []
db.each(
"SELECT * FROM releases ORDER BY id DESC",
(err, release) => {
release.links = []
db.all("SELECT url FROM links WHERE id = ?", [release.id], (err, links) => {
links = links.map((e) => { return e.url })
release.links = links
// line above: tried
// links.forEach((e) => { release.links.push(e.url) })
// too, but that didn't work either.
})
all.push(release)
},
(complete) => { console.log(all) }
)
}
main()
Though, when I run it, it inevitably shows:
links: []
Every time. How can I fix this?
Thank you in advance.
Edit 1:
This SQL snippet generates the database, and populates it with some data.
CREATE TABLE `links` ( `id` TEXT, `url` TEXT );
CREATE TABLE `releases` ( `id` TEXT, `title` TEXT, `image` TEXT );
INSERT INTO links VALUES
('rel-001', 'https://example.com/mirror1'),
('rel-001', 'https://example.com/mirror2');
INSERT INTO releases VALUES
('rel-001', 'Release 001', 'https://example.com/image.jpg');
The goal is to have something like this:
{
releases:[
{
id:'rel-001',
title:'Release 001',
image:'https://example.com/image.jpg',
links:[
'https://example.com/mirror1',
'https://example.com/mirror2'
]
}
]
}

Answer 1

try to see if both queries are being executed by adding console.log in the callbacks, moreover you should push the links only within the second callback since before the callback is fired the value is not existing, thus you are trying to push an empty value, also you don't need to initialize release.links = [], all will be only filled after all queries are executed, so therefore we need to execute console.log(all); in the last child callback:
function main() {
all = []
var parentComplete = false;
db.each("SELECT * FROM releases ORDER BY id DESC", (err, release) => {
db.all("SELECT url FROM links WHERE id = ?", [release.id], (err, links) => {
release.links = links.map(e => e.url);
all.push(release);
if (parentComplete){
console.log(all);
}
})
},
(complete) => {
parentComplete = true;
}
)
}
main();
p.s. in order to get the result you want you will need to initialize all as an object all = {releases:[]}
function main() {
all = {releases:[]};
var parentComplete = false;
db.each("SELECT * FROM releases ORDER BY id DESC", (err, release) => {
db.all("SELECT url FROM links WHERE id = ?", [release.id], (err, links) => {
release.links = links.map(e => e.url);
all.releases.push(release);
if (parentComplete){
console.log(all);
}
})
},
(complete) => {
parentComplete = true;
}
)
}
main();