Fetching id field of an entity in the SQL Transaction - postgresql

Question

I have the following schema.
Person(pid, pname)
Beer(bid, bname)
Likes(pid,bid)
I would like to insert a likes item. However, I am accepting the following format for the new users : (Pid, pname, bid, bname).
I would like to create a transaction for that to avoid conflict ( This is a highly simplified version of my real problem but the issue is the same). In my Person table, I set pid Auto-Increment(or Serial in Postgresql). Also the same goes for bid.
I have stuck in a point where I know the Person does not exist but the beer exists. So, I have to create a Person, then add an entity to Likes relation.
As far as I know, when I use the Autocommit(false) in dB, the transaction won't save until the commit. So, should I change the db design:
Change the auto-increment field to a normal integer, not null field.
In the transaction, after the autoCommit(false) has begun, read the last entry of the person
Increment it by one while creating the new person
Then create likes relation
Or, is there any other way around or do I miss something about transactions?
Here is what I have done so far:
try {
String add_person_sql = "INSERT INTO Person (name) VALUES(?)";
PreparedStatement add_person_statement = mydb.prepareStatement(add_person_sql);
String add_likes_sql = "INSERT INTO Likes (pid, bid) VALUES(?, ?)";
PreparedStatement add_likes_statement = mydb.prepareStatement(add_likes_sql);
mydb.setAutoCommit(false);
add_person_statement.setString(1, pname);
// The problem is, without saving the person I cannot know the id of the person
// AFAIK, this execution is not finished until commit occurs
add_person_statement.executeQuery();
// How can I fetch person's id
add_likes_statement.setString(1, pid);
add_likes_statement.setString(2, bid);
add_likes_statement.executeQuery();
mydb.commit();
}
catch(Exception e){
System.out.println(e);
mydb.rollback();
}

Answer 1

You can tell JDBC to return the generated ID from the insert statement, then you can use that ID to insert into the likes table:
mydb.prepareStatement(add_person_sql, new String[]{"pid"});
The second parameter tells the driver to return the generated value for the pid column.
Alternatively you can use
mydb.prepareStatement(add_person_sql, Statement.RETURN_GENERATED_KEYS);
that tells the driver to detect the auto increment columns.
Then run the insert using executeUpdate()
add_person_statement.setString(1, pname);
add_person_statement.executeUpdate();
int newPid = -1;
ResultSet idResult = add_person.getGeneratedKeys();
if (idResult.next()) {
newPid = idResult.getInt(1);
}
add_likes_statement.setString(1, newPid);
add_likes_statement.setString(2, bid);
add_likes_statement.executeUpdate();
mydb.commit();