Question

所以我使用FMDB库，我想启用通过

完成的外键

[db executeQuery:@"PRAGMA foreign_keys=ON"];

但我使用FMDatabaseQueue，初始化为

-(FMDatabaseQueue*)queue
{

    if (_queue == nil)
    {
        FMDatabaseQueue* queue = [FMDatabaseQueue databaseQueueWithPath:self.dbPath];
        _queue = queue;
    }
    return _queue;
}

然后我就像这样使用它

-(NSNumber*)phoneDBID:(NSString*)phoneNumber
{
    __block NSNumber* phoneDBID = nil;
    [self.queue inDatabase:^(FMDatabase *db) {
        FMResultSet* result = [db executeQuery:@"SELECT * from Contact_numbers where number= ?;", phoneNumber];
        if ([result next])
        {
            phoneDBID = [NSNumber numberWithInt:[result intForColumn:@"contact_id"]];
        }
        [result close];
    }];
    return phoneDBID;
}

并且我不认为外键是在队列中启用的，有没有办法如何为队列启用它们，或者我必须在每个查询中执行它？

Answer 1

两个观察结果：

关于使用FMDatabaseQueue的外键约束的唯一警告是，我建议不要在PRAGMA foreign_keys事务中使用FMDatabaseQueue（即在inTransaction块内）。 PRAGMA foreign_keys的{{3}}说：

此pragma是交易中的无操作;只有在没有待处理的BEGIN或SAVEPOINT时，才能启用或禁用外键约束强制执行。

如果你从inDatabase块内执行此编译指示，那么你会没事的。

您的示例并未说明foreign_keys的实际效果。外键约束仅在修改数据库中的数据时适用。但您只是SELECT，因此PRAGMA foreign_keys的设置无关紧要。

为了说明此pragma的使用，请考虑以下示例。我创建了book和author个表，前者有后者的外键：

[queue inDatabase:^(FMDatabase *db) {
    success = [db executeUpdate:@"create table author (author_id integer primary key, name text)"];
    if (!success) NSLog(@"Create author table failed: %@", [db lastErrorMessage]);

    success = [db executeUpdate:@"create table book (book_id integer primary key, author_id integer, title text, FOREIGN KEY(author_id) REFERENCES author(author_id))"];
    if (!success) NSLog(@"Create book table failed: %@", [db lastErrorMessage]);
}];

如果没有foreign_keys pragma，则可以：

[queue inDatabase:^(FMDatabase *db) {

    // without foreign key constraints enforced, this will succeed, even though the author_id has not yet been added to author table

    success = [db executeUpdate:@"insert into book (book_id, author_id, title) values (?, ?, ?)", @(1), @(101), @"Romeo and Juliet"];
    if (!success) NSLog(@"Insert 'Romeo and Juliet' failed: %@", [db lastErrorMessage]);

    // obviously, this will, too

    success = [db executeUpdate:@"insert into author (author_id, name) values (?, ?)", @(101), @"William Shakespeare"];
    if (!success) NSLog(@"Insert 'William Shakespeare' failed: %@", [db lastErrorMessage]);
}];

但如果我打开外键：

[queue inDatabase:^(FMDatabase *db) {

    // turn on foreign keys

    success = [db executeUpdate:@"PRAGMA foreign_keys = YES"];
    if (!success) NSLog(@"Foreign keys pragma failed: %@", [db lastErrorMessage]);
}];

如果再试一次，book的第一次插入没有相应的author条目就会失败。在插入book条目之前，我无法插入author条目：

[queue inDatabase:^(FMDatabase *db) {

    // with foreign key this should (and does) fail

    success = [db executeUpdate:@"insert into book (book_id, author_id, title) values (?, ?, ?)", @(2), @(201), @"One Hundred Years of Solitude"];
    if (!success) NSLog(@"First insert of 'Hundred Years of Solitude' failed: %@", [db lastErrorMessage]);

    // but if we insert author ...

    success = [db executeUpdate:@"insert into author (author_id, name) values (?, ?)", @(201), @"Gabriel García Márquez"];
    if (!success) NSLog(@"Insert 'Gabriel García Márquez' failed: %@", [db lastErrorMessage]);

    // ... now this will succeed.

    success = [db executeUpdate:@"insert into book (book_id, author_id, title) values (?, ?, ?)", @(2), @(201), @"One Hundred Years of Solitude"];
    if (!success) NSLog(@"Second insert 'Hundred Years of Solitude' failed: %@", [db lastErrorMessage]);
}];

所以，它的长短不一样，外键可以与FMDatabaseQueue一起正常工作，但我只建议不要在inTransaction调用中做到这一点。

FMDatabaseQueue中的外键

1 个答案: