在Core Data中按属性获取对象 [英] Fetch object by property in Core Data
问题描述
在我的iPhone项目中,我想写一个函数,检查在我的Core Data ManagedObjectContext中有一个对象的某个属性的给定值,如 some_property
。
In my iPhone project, I want to write a function that checks wether there's an object in my Core Data ManagedObjectContext with a given value for a certain property, say some_property
.
如果已经有一个对象 some_property == 12
,我想让函数返回对象,否则,我想创建对象,或者至少返回 nil
。
If there's already an object with some_property == 12
, I want the function to return the object, otherwise, I want to create the object, or at least return nil
.
我该怎么做?
推荐答案
以下代码段显示了如何检索与特定谓词匹配的对象。如果没有这样的对象,该代码段显示如何创建一个新对象,保存并返回。
The following snippet shows how to retrieve the objects matching a specific predicate. If there are no such objects, the snippet shows how to create a new object, save it and return it.
NSFetchRequest *request = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"YourEntityName" inManagedObjectContext:managedObjectContext];
[request setEntity:entity];
// retrive the objects with a given value for a certain property
NSPredicate *predicate = [NSPredicate predicateWithFormat: @"property == %@", value];
[request setPredicate:predicate];
// Edit the sort key as appropriate.
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"yourSortKey" ascending:YES];
NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];
[request setSortDescriptors:sortDescriptors];
// Edit the section name key path and cache name if appropriate.
// nil for section name key path means "no sections".
NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:request managedObjectContext:managedObjectContext sectionNameKeyPath:nil cacheName:@"Root"];
aFetchedResultsController.delegate = self;
NSError *error = nil;
NSArray *result = [managedObjectContext executeFetchRequest:request error:&error];
[request release];
[sortDescriptor release];
[sortDescriptors release];
if ((result != nil) && ([result count]) && (error == nil)){
return [NSMutableArray arrayWithArray:result];
}
else{
YourEntityName *object = (YourEntityName *) [NSEntityDescription insertNewObjectForEntityForName:@"YourEntityName" inManagedObjectContext:self.managedObjectContext];
// setup your object attributes, for instance set its name
object.name = @"name"
// save object
NSError *error;
if (![[self managedObjectContext] save:&error]) {
// Handle error
NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
}
return object;
}
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