Node js SQL Transactions REST - sql

Question

I have implemented a REST API with express.js. I use it to connect to my database. The database has to tables. One is the table Person and the other is called Pet.
app.post('/persons', (req, res, next) => {
let firstname = req.body.firstname;
let lastname = req.body.lastname;
let petname = req.body.petname;
if (!firstname) {
return res.status(400).send({ error: true, message: 'Please provide first name' });
}else if (!lastname) {
return res.status(400).send({ error: true, message: 'Please provide last name' });
}else if (!petname) {
return res.status(400).send({ error: true, message: 'Please provide pet name' });
}
When i call this post method i want to check if a certain petname already exists in the database. If so, then get the petID and insert it with the first name and the last name in the table Person, so that a person is linked with a pet. If this petname does not exist, then create a new pet with this name and a new id in the table Pet. Then again save the id and the first name and last name to the table Person. So every petname should only exist once.
I know how to write the Person into the database:
Conn.query("INSERT INTO Person SET ? ", { FirstName: firstname, LastName: lastname, PetID: petid }, function (error, results, fields) {
if (error) throw error;
return res.send({ error: false, data: results, message: 'New person has been created successfully.' });
});
But now i still need the petID, if available, and if not create a new pet in the DB and return the id.
How do i do that?

Answer 1

I think checking before creating transaction is your option.
If pet exists you will get it's id, and if not - create new pet and also get the id.
I wrote a small sql script for psql:
WITH pet AS (
WITH new_row AS (
INSERT INTO Pets (id)
SELECT '{id}' WHERE NOT EXISTS (SELECT * FROM Pets WHERE id = '{id}')
RETURNING *
)
SELECT * FROM new_row
UNION
SELECT * FROM Pets WHERE id = '{id}' LIMIT 1
RETURNING *
)
INSERT INTO Person SET pet_id=pet.id;
SOURCE FROM RELATED QUESTION
Next step is creating transaction for person. Now you already have your pet id.

Answer 2

So i tried this but it does not work.
Conn.beginTransaction(function(error){
if (error) throw error;
Conn.query('Select PetID FROM Pet WHERE Petname = ' + mysql.escape(petname), function (error, results, fields){
if (error) {
connection.rollback(function() {
throw error;
});
}
else {
if (!results) {
Conn.query('INSERT INTO Pet SET ? ', { Petname: petname }, function (error, results, fields){
if (error) {
connection.rollback(function() {
throw err;
});
}
return res.send({ error: false, data: results, message: 'New pet has been created successfully.' });
});
}
let petid = results.insertId;
Conn.query('INSERT INTO Person SET ? ', { FirstName: firstname, LastName: lastname, PetID: petid}, function (error, results, fields){
if (error) {
connection.rollback(function() {
throw error;
});
}
Conn.commit;
return res.send({ error: false, data: results, message: 'New person has been created successfully.' });
});
}
});
});
What is wrong with this transaction?