Drupal 7 db_update - checking for errors - php

Question

I have a custom Drupal 7 module with an 'edit' page.
The form fields reference a couple of database tables, so to process the form, we attempt to update the first table, and we try to set a '$error' to 'true' and check against $error before we attempt to update the next table. For example:
HTML:
<input name="field1" />
<input name="field2" />
PHP:
$error = false;
$update_table_1 = db_update('table1')
->fields(array(
'field1' => $_POST['field1'],
))
->condition('id', $id)
->execute();
if(!update_table_1) {
$error = true;
}
if(!$error) {
$update_table_2 = db_update('table2')
->fields(array(
'field2' => $_POST['field2'],
))
->condition('id', $id)
->execute();
if(!$update_table_2) {
$error = true;
}
}
Problem: If only updating something in table 2, it will throw an error before it event gets to update table 2 because db_query says that it is not true since the field was the same as what was in the database (no change). Really, I only want to stop it if there was a database / code error.
Does the Drupal 7 db_update API have some kind of error reporting function like mysql_error()? Other suggestions?

Answer 1

The safest way you can do it is with a transaction and proper PHP error checking:
$transaction = db_transaction();
try {
// Query 1
db_update(...);
// Query 2
db_update(...);
}
catch (Exception $e) {
// Rollback the transaction
$transaction->rollback();
// Do something with the exception (inform user, etc)
}
I should mention the transaction is only necessary if you don't want the changes from the first query to persist if the second query fails. It's quite a common requirement but might not fit your use case.