F＃树：节点插入(F# Tree: Node Insertion)

 我很快检查了PRAGMA foreign_keys = ON; 在5.1模拟器和5.1 iPod Touch上都可以正常工作。 正如ccgus所建议的那样，您应该缓存数据库连接。 如果使用队列，只需缓存队列并重新组织代码，这样每次需要使用数据库时都不会创建新的队列。 用你目前的方法，如果你没有真正使用它，而是每次都创造新的东西，那么拥有一个队列有什么意义？ 
 
 但是回到问题中，正如您已经知道的那样，外键默认是关闭的，所以您需要先启用它。 我设法用PRAGMA foreign_keys = ON;来做到这一点PRAGMA foreign_keys = ON; ，这里是我使用的更多的测试代码： 
 
//create database
NSString* dbPath = [(NSArray*)NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) objectAtIndex:0];
dbPath = [dbPath stringByAppendingPathComponent:@"test.db"];
db = [FMDatabase databaseWithPath:dbPath];
if ([db open]) {
    NSLog(@"Database %@ opened", dbPath);
    //check for foreign_key
    NSString* sql = @"PRAGMA foreign_keys";
    FMResultSet *rs = [db executeQuery:sql];
    int enabled;
    if ([rs next]) {
        enabled = [rs intForColumnIndex:0];
    }
    [rs close];
    if (!enabled) {
        // enable foreign_key
        sql = @"PRAGMA foreign_keys = ON;";
        [db executeUpdate:sql];
        // check if successful
        sql = @"PRAGMA foreign_keys";
        FMResultSet *rs = [db executeQuery:sql];
        if ([rs next]) {
            enabled = [rs intForColumnIndex:0];
        }
        [rs close];
    }
    // do your stuff here, or just cache the connection
} else {
    NSLog(@"Failed to open %@", dbPath);
}
 
 看起来很简单，唯一想到的是你使用executeQuery而不是executeUpdate 。 

I quickly checked and PRAGMA foreign_keys = ON; works fine for me both on 5.1 simulator and 5.1 iPod Touch. As ccgus suggests, you should cache the data base connection. If you use the queue, just cache the queue and reorganize your code so it doesn't create new queue each time you need to use the database. With your current approach, what is the point of having a queue if you don't really use it but create new each time?
 
But back to the question, as you are already aware, foreign keys are off by default, so you need to first enable it. I managed to do it with PRAGMA foreign_keys = ON;, here is some more of the test code I used:
 
//create database
NSString* dbPath = [(NSArray*)NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) objectAtIndex:0];
dbPath = [dbPath stringByAppendingPathComponent:@"test.db"];
db = [FMDatabase databaseWithPath:dbPath];
if ([db open]) {
    NSLog(@"Database %@ opened", dbPath);
    //check for foreign_key
    NSString* sql = @"PRAGMA foreign_keys";
    FMResultSet *rs = [db executeQuery:sql];
    int enabled;
    if ([rs next]) {
        enabled = [rs intForColumnIndex:0];
    }
    [rs close];
    if (!enabled) {
        // enable foreign_key
        sql = @"PRAGMA foreign_keys = ON;";
        [db executeUpdate:sql];
        // check if successful
        sql = @"PRAGMA foreign_keys";
        FMResultSet *rs = [db executeQuery:sql];
        if ([rs next]) {
            enabled = [rs intForColumnIndex:0];
        }
        [rs close];
    }
    // do your stuff here, or just cache the connection
} else {
    NSLog(@"Failed to open %@", dbPath);
}
 
Looks fairly straightforward, the only thing that comes to mind is that you used executeQuery instead of executeUpdate.