iOS SQLite FMDB Transactions.. Correct usage? - objective-c

Question

I'm just going to try out using transactions with the FMDB SQLite iOS wrapper.
The documentation is a little vague on transactions but from having a quick look at some functions I have come up with the following logic:
[fmdb beginTransaction];
// Run the following query
BOOL res1 = [fmdb executeUpdate:#"query1"];
BOOL res2 = [fmdb executeUpdate:#"query2"];
if(!res1 || !res2) [fmdb rollback];
else [fmdb commit];

Answer 1

You could also use FMDatabaseQueue to handle your transactions, which is part of fmdb:
[queue inTransaction:^(FMDatabase *db, BOOL *rollback) {
[db executeUpdate:#"INSERT INTO myTable VALUES (?)", [NSNumber numberWithInt:1]];
[db executeUpdate:#"INSERT INTO myTable VALUES (?)", [NSNumber numberWithInt:2]];
[db executeUpdate:#"INSERT INTO myTable VALUES (?)", [NSNumber numberWithInt:3]];
if (whoopsSomethingWrongHappened) {
*rollback = YES;
return;
}
// etc…
[db executeUpdate:#"INSERT INTO myTable VALUES (?)", [NSNumber numberWithInt:4]];
}];
Documentation

Answer 2

I wouldn't try to do the second update if the first failed.
bool ret = false;
[fmdb beginTransaction];
ret = [fmdb executeUpdate:#"query1"];
if (ret)
{
ret = [fmdb executeUpdate:#"query2"];
if (!ret)
{
// report error 2
}
}
if(ret)
{
if (![fmdb commit])
{
// panic!
}
}
else
{
if (![fmdb rollback])
{
// panic!
}
}
For paranoid robustness you should have a try ... catch block in case anything throws an exception. If you do, you can use it to your advantage.
[fmdb beginTransaction];
#try
{
if (![fmdb executeUpdate:#"query1"])
{
// report error
#throw someExcpetion;
}
if (![fmdb executeUpdate:#"query2"])
{
// report error
#throw someExcpetion;
}
[fmdb commit]
}
#catch(NSException* e)
{
[fmdb rollback];
// rethrow if not one of the two exceptions above
}

Answer 3

Swift way:
let queue = FMDatabaseQueue(path: databaseURL.path!)
queue.inTransaction() {
db, rollback in
result = db.executeUpdate("INSERT INTO client VALUES (NULL, ?)", client.name ?? "")
if result {
client.ID = Int(db.lastInsertRowId())
} else {
rollback.initialize(true)
print("\(__FUNCTION__) insert into table failed: \(db.lastErrorMessage())")
}
}
queue.close()

Answer 4

It seems like a valid usage scenario, to which I might add outputting the values of -lastErrorMessage and -lastErrorCode before you perform a rollback, so that you get a sense of what exactly went wrong.
Better yet, make those calls after each -executeUpdate, so you'll know if an error occured after each statement:
[fmdb beginTransaction];
// Run the following query
BOOL res1 = [fmdb executeUpdate:#"query1"];
if (!res1) {
NSLog(#"Error %d - %#", [fmdb lastErrorMessage], [fmdb lastErrorCode]);
}
BOOL res2 = [fmdb executeUpdate:#"query2"];
if (!res2) {
NSLog(#"Error %d - %#", [fmdb lastErrorMessage], [fmdb lastErrorCode]);
}
if(!res1 || !res2) [fmdb rollback];
else [fmdb commit];