objective-c - 将 NSManagedObjectModel 中的关系添加到以编程方式创建的 NSEntityDescription

Question

当你编写一个使用 CoreData 的静态库时，将一个普通的 .xdatamodeld 文件包含到项目中会是一团糟，因为你不能简单地将它的编译版本 (.momd) 链接到你的二进制文件中，所以最好创建整个 NSManagedObjectModel 在代码中是这样的:

NSAttributeDescription *dateAttribute = NSAttributeDescription.new;

dateAttribute.name = @"timestamp";
dateAttribute.attributeType = NSDoubleAttributeType;
dateAttribute.optional = NO;
dateAttribute.indexed = YES;

NSAttributeDescription *payloadAttribute = NSAttributeDescription.new;

payloadAttribute.name = @"payload";
payloadAttribute.attributeType = NSBinaryDataAttributeType;
payloadAttribute.optional = NO;
payloadAttribute.indexed = NO;

NSEntityDescription *entry = NSEntityDescription.new;

entry.name = entry.managedObjectClassName = NSStringFromClass(MyCustomEntry.class);
entry.properties = @[dateAttribute, payloadAttribute];

NSManagedObjectModel *mom = NSManagedObjectModel.new;

mom.entities = @[entry];

一切都很完美....

但是!等等，如果我的 NSManagedObjectModel 中有多个实体并且它们相关(对多、反向等)，那么我将如何在代码中连接它们，就像示例中那样上面，没有那个漂亮的 Xcode 编辑器，你在哪里通过几次鼠标点击建立关系？

示例

想象一下，我们有一个类 MyCustomElement，它与上面代码中的 MyCustomEntry 几乎相同。现在，如果我为实体使用 Xcode 生成，它们的界面如下:

@interface MyCustomEntry : NSManagedObject

@property (nonatomic, retain) NSNumber *timestamp;
@property (nonatomic, retain) NSData *payload;
@property (nonatomic, retain) MyCustomElement *element;

@end

@interface MyCustomElement : NSManagedObject

@property (nonatomic, retain) NSNumber * timestamp;
@property (nonatomic, retain) NSString * identifier;
@property (nonatomic, retain) NSSet *entries;

@end

@interface MyCustomElement (CoreDataGeneratedAccessors)

- (void)addEntriesObject:(MyCustomEntry *)value;
- (void)removeEntriesObject:(MyCustomEntry *)value;
- (void)addEntries:(NSSet *)values;
- (void)removeEntries:(NSSet *)values;

@end

我需要为他们创建什么 NSRelationshipDescription 以及如何初始化它？

Answer 1

关系由 NSRelationshipDescription 对象描述。以下代码为“MyCustomEntry”、“MyCustomElement”创建两个具有关系的实体描述

• 条目(MyCustomElement --> MyCustomEntry，对多)，
• element(MyCustomEntry --> MyCustomElement，一对一)，entries 的倒数。

两个实体都只有一个字符串属性“identifier”(为了节省一些代码行)。

objective-c :

NSEntityDescription *entry = [[NSEntityDescription alloc] init];
[entry setName:@"MyCustomEntry"];
[entry setManagedObjectClassName:@"MyCustomEntry"];

NSAttributeDescription *entryIdAttribute = [[NSAttributeDescription alloc] init];
entryIdAttribute.name = @"identifier";
entryIdAttribute.attributeType = NSStringAttributeType;

NSEntityDescription *element = [[NSEntityDescription alloc] init];
[element setName:@"MyCustomElement"];
[element setManagedObjectClassName:@"MyCustomElement"];

NSAttributeDescription *elementIdAttribute = [[NSAttributeDescription alloc] init];
elementIdAttribute.name = @"identifier";
elementIdAttribute.attributeType = NSStringAttributeType;

// To-many relationship from "Element" to "Entry":
NSRelationshipDescription *entriesRelation = [[NSRelationshipDescription alloc] init];

// To-one relationship from "Entry" to "Element":
NSRelationshipDescription *elementRelation = [[NSRelationshipDescription alloc] init];

[entriesRelation setName:@"entries"];
[entriesRelation setDestinationEntity:entry];
[entriesRelation setMinCount:0];
[entriesRelation setMaxCount:0]; // max = 0 for to-many relationship
[entriesRelation setDeleteRule:NSCascadeDeleteRule];
[entriesRelation setInverseRelationship:elementRelation];

[elementRelation setName:@"element"];
[elementRelation setDestinationEntity:element];
[elementRelation setMinCount:0];
[elementRelation setMaxCount:1]; // max = 1 for to-one relationship
[elementRelation setDeleteRule:NSNullifyDeleteRule];
[elementRelation setInverseRelationship:entriesRelation];

[entry setProperties:@[entryIdAttribute, elementRelation]];
[element setProperties:@[elementIdAttribute, entriesRelation]];

NSManagedObjectModel *mom = [[NSManagedObjectModel alloc] init];
[mom setEntities:@[entry, element]];

Swift(现已针对 Swift 3/4 更新):

let entry = NSEntityDescription ()
entry.name = "MyCustomEntry"
entry.managedObjectClassName = "MyCustomEntry"

let entryIdAttribute = NSAttributeDescription()
entryIdAttribute.name = "identifier";
entryIdAttribute.attributeType = .stringAttributeType;

let element = NSEntityDescription()
element.name = "MyCustomElement"
element.managedObjectClassName = "MyCustomElement"

let elementIdAttribute =  NSAttributeDescription()
elementIdAttribute.name = "identifier"
elementIdAttribute.attributeType = .stringAttributeType

// To-many relationship from "Element" to "Entry":
let entriesRelation = NSRelationshipDescription()

// To-one relationship from "Entry" to "Element":
let elementRelation = NSRelationshipDescription ()

entriesRelation.name = "entries"
entriesRelation.destinationEntity = entry
entriesRelation.minCount = 0
entriesRelation.maxCount = 0  // max = 0 for to-many relationship
entriesRelation.deleteRule = .cascadeDeleteRule
entriesRelation.inverseRelationship = elementRelation

elementRelation.name = "element"
elementRelation.destinationEntity = element
elementRelation.minCount = 0
elementRelation.maxCount = 1 // max = 1 for to-one relationship
elementRelation.deleteRule = .nullifyDeleteRule
elementRelation.inverseRelationship = entriesRelation

entry.properties = [entryIdAttribute, elementRelation]
element.properties = [elementIdAttribute, entriesRelation]

let mom = NSManagedObjectModel()
mom.entities = [entry, element]

我已经测试了这段代码，它似乎可以工作，所以我希望它对你有用。