xcode 我们可以在核心数据中设置主键吗
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can we set primary key in core data
提问by mrugen
I am making a application in which i use core data . Now in this i need to create a primary key same as we create in sqlite (auto increment type ). Is this possible with core data .If yes then how any link or suggestion so as how to proceed for the same.
我正在制作一个使用核心数据的应用程序。现在，我需要创建一个与我们在 sqlite 中创建的主键相同的主键（自动增量类型）。这是否可能与核心数据。如果是，那么任何链接或建议如何进行相同的操作。
Regards Mrugen
问候姆鲁根
采纳答案by Peter DeWeese
Since core data is a persistance framework more than a general purpose database, it abstracts the primary key.
由于核心数据是一个持久性框架，而不是通用数据库，因此它抽象了主键。
回答by Shekhar Gupta
CoreData isn't a Database rather its an Object Relational Mapping Solution. It's an object persistence layer. There is no concept of primary keys or foreign keys in CoreData.
CoreData 不是数据库，而是一个对象关系映射解决方案。它是一个对象持久层。CoreData 中没有主键或外键的概念。
If you want to establish a relationship between two entities. You'll define a relationship, CoreData takes care of how that relationship is stored.
如果要在两个实体之间建立关系。您将定义一个关系，CoreData 负责该关系的存储方式。
Select an entity, use plus button at the bottom of the entities attributes list, select add relationship, select the destination entity from the dropdown.
选择一个实体，使用实体属性列表底部的加号按钮，选择添加关系，从下拉列表中选择目标实体。
Select the destination entity and define an inverse relationship in the same way.
选择目标实体并以相同方式定义反向关系。
回答by Paul Hunter
Couldn't you just use the objectID property of NSManagedObject? This would give you a unique reference to your object.
你不能只使用 NSManagedObject 的 objectID 属性吗？这将为您提供对您的对象的唯一引用。
回答by karim
The answer is NO. To achieve similar thing, You can have an 'id' field in the table, before inserting into the table get the maximum value of the 'id' and then add 1 to the value.
答案是不。为了实现类似的事情，您可以在表中有一个 'id' 字段，在插入表之前获取 'id' 的最大值，然后将该值加 1。
With a filed 'userID' in the table ENTITY_USER (users), the get the highest user ID.
使用表 ENTITY_USER (users) 中的归档“userID”，获得最高的用户 ID。
+ (NSInteger) getMaxUserID
{
NSFetchRequest *request = [[NSFetchRequest alloc] init];
NSEntityDescription *res = [NSEntityDescription entityForName:ENTITY_USER
inManagedObjectContext:[DataBaseManager sharedInstance].managedObjectContext];
[request setEntity:res];
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"userID" ascending:NO];
NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];
[request setSortDescriptors:sortDescriptors];
[sortDescriptors release];
[sortDescriptor release];
[request setFetchLimit:1];
NSError *error = nil;
NSArray *results = [[DataBaseManager sharedInstance].managedObjectContext executeFetchRequest:request error:&error];
[request release];
if (results == nil) {
NSLog(@"error fetching the results: %@",error);
}
NSInteger maximumValue = 0;
if (results.count == 1) {
CDUser *result = (CDUser *)[results objectAtIndex:0];
maximumValue = [result.userID integerValue];
}
return maximumValue;
}
