我查看了许多谓词问题，我阅读了文档，但似乎没有什么可以作为我的问题的答案。
我有一个名为 Materials 的核心数据实体，我有属性 category、subcategory 和 description。
我有三个 UITableViewControllers 并且在每个中我想使用一个谓词来显示如下:
TableViewController 1:只有类别(类别名称不重复)
选择一个类别并转到 TableViewController 2。
TableViewController 2:显示子类别(不重复子类别名称)
选择一个子类别并转到 TableViewController 3 列出该类别和子类别中的所有项目。
如果不在核心数据模型中使用三个实体，我可以这样做吗？
我尝试在我的 fetchedResultsController 方法中使用以下谓词代码，但成功了:
Materials * materials = [[Materials alloc]init];//this doesn't feel like it belongs inside a fetchedResultsController
NSPredicate * predicate = [NSPredicate predicateWithFormat
"category == %@", materials.category];
fetchRequest.predicate = predicate;
这是我第一次尝试以这种方式排序和显示，我通常会按照惯例使用关系谓词，但为一组数据( Material )设置 3 个实体似乎不合逻辑。
您不需要为每个 Material 、类别和子类别设置三个不同的 NSMO。您只需要一个 NSMO，它是具有这些属性类别、子类别和描述的 Material 。
在 ViewController 1 中显示类别:
NSFetchRequest *fetchRequest = [NSFetchRequest fetchRequestWithEntityName"Materials"];
NSEntityDescription *entity = [NSEntityDescription entityForName"Materials" inManagedObjectContext:self.managedObjectContext];
// Required! Unless you set the resultType to NSDictionaryResultType, distinct can't work.
// All objects in the backing store are implicitly distinct, but two dictionaries can be duplicates.
// Since you only want distinct Categories, only ask for the 'Category' property.
fetchRequest.resultType = NSDictionaryResultType;
fetchRequest.propertiesToFetch = [NSArray arrayWithObject:[[entity propertiesByName] objectForKey"Category"]];
fetchRequest.returnsDistinctResults = YES;
// Now it should yield an NSArray of distinct values in dictionaries.
NSArray *dictionaries = [self.managedObjectContext executeFetchRequest:fetchRequest error:nil];
NSLog (@"names: %@",dictionaries);
通过它，您可以从核心数据中获取具有不同类别的所有 Material NSManagedObject。
用于在 ViewController 2 中显示子类别:
NSFetchRequest *fetchRequest = [NSFetchRequest fetchRequestWithEntityName"Materials"];
NSEntityDescription *entity = [NSEntityDescription entityForName"Materials" inManagedObjectContext:self.managedObjectContext];
NSPredicate * predicate = [NSPredicate predicateWithFormat"SELF.category == %@", selectedCategoryName];
fetchRequest.predicate = predicate;
// Required! Unless you set the resultType to NSDictionaryResultType, distinct can't work.
// All objects in the backing store are implicitly distinct, but two dictionaries can be duplicates.
// Since you only want distinct SubCategory, only ask for the 'SubCategory' property.
fetchRequest.resultType = NSDictionaryResultType;
fetchRequest.propertiesToFetch = [NSArray arrayWithObject:[[entity propertiesByName] objectForKey"SubCategory"]];
fetchRequest.returnsDistinctResults = YES;
// Now it should yield an NSArray of distinct values in dictionaries.
NSArray *dictionaries = [self.managedObjectContext executeFetchRequest:fetchRequest error:nil];
NSLog (@"names: %@",dictionaries);
通过它，您可以从核心数据中获取具有不同类别的所有 Material NSManagedObject。
让第三个 Viewcontroller 列出属于 Selected 类别和子类别的所有项目:
NSFetchRequest *fetchRequest = [NSFetchRequest fetchRequestWithEntityName"Materials"];
NSEntityDescription *entity = [NSEntityDescription entityForName"Materials" inManagedObjectContext:self.managedObjectContext];
NSPredicate * predicate = [NSPredicate predicateWithFormat:@"SELF.category == %@ and SELF.subcategory == %@", selectedCategoryName,selectedSubcatgory];
fetchRequest.predicate = predicate;
// Required! Unless you set the resultType to NSDictionaryResultType, distinct can't work.
// All objects in the backing store are implicitly distinct, but two dictionaries can be duplicates.
// Now it should yield an NSArray of distinct values in dictionaries.
NSArray *dictionaries = [self.managedObjectContext executeFetchRequest:fetchRequest error:nil];
NSLog (@"names: %@",dictionaries);
关于ios - 显示核心数据 : If attribute has same name display once，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/27538930/
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