FMDatabaseQueue 中的外键

Question

所以我正在使用 FMDB 库，我想启用通过

完成的外键

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

但我使用的是 FMDatabaseQueue，像这样初始化

-(FMDatabaseQueue*)queue
{

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

然后我就这样使用它

-(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;
}

我认为队列中没有启用外键，有没有办法为队列启用它们，或者我必须在每个查询中都这样做？

Answer 1

两个观察结果：

1. 关于FMDatabaseQueue 的外键约束的唯一警告是，我建议不要在FMDatabaseQueue 事务中（即在inTransaction 块中）使用PRAGMA foreign_keys。 The documentation for PRAGMA foreign_keys 说：

   此编译指示是事务中的空操作；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。

   不过，如果您在 inDatabase 块内执行此编译指示，您会没事的。

2. 您的示例并未说明foreign_keys 的实际应用。外键约束仅在修改数据库中的数据时适用。但是你只是在做SELECT，所以PRAGMA foreign_keys的设置无关紧要。

   为了说明这个 pragma 的用法，请考虑以下示例。我创建了book 和author 表，其中前者具有后者的外键：

   [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 编译指示，这可行：

   [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]);
   }];
   

   但是如果我打开外键：

   [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 条目会失败。在插入 author 条目之前，我无法插入 book 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]);
   }];
   

因此，总而言之，外键与FMDatabaseQueue 配合使用都很好，但我只建议不要在inTransaction 调用中这样做。