iOS，每次保存上下文时，核心数据都会将新对象添加到错误的实体中？

Question

我是核心数据新手。在这里，我试图在一个月内保存一堆语句。但我不知道如何在具有关系的核心数据中插入对象。我的代码正在做什么，它在 Month 实体中保存月份，并在 Statement 实体中保存语句，没有任何关系。每次我保存它时，都会在 Month 实体中创建新对象。我的数据模型有一对多关系。例如，当前(月份)11 月，我想插入一些与 11 月相关的声明。比下个月的 12 月，我想插入一些与 12 月相关的声明。这就是我想要实现的目标。抱歉我的英语不好。
`

NSManagedObjectContext *context = [self managedObjectContext];

NSEntityDescription *entity = [NSEntityDescription entityForName:@"Month" inManagedObjectContext:context];
Month *month = [[Month alloc] initWithEntity:entity insertIntoManagedObjectContext:context];

Statement *statement = [NSEntityDescription insertNewObjectForEntityForName:@"Statement" inManagedObjectContext:context];
statement.title = self.enterTextGreenViewTextField.text;

NSNumber  *aNum = [NSNumber numberWithDouble:savingAmount];
statement.amount =aNum;
statement.isIncome = [NSNumber numberWithBool:isIncome];

[month addStatementsObject:statement];

NSError *error;
if (![context save:&error]) {
    FATAL_CORE_DATA_ERROR(error);
    return;
}

NSLog(@"savingStatement ****");

`

#import "Month.h"

NS_ASSUME_NONNULL_BEGIN

@interface Month (CoreDataProperties)

@property (nullable, nonatomic, retain) NSDate *monthOfStatement;
@property (nullable, nonatomic, retain) NSSet<Statement *> *statements;

@end

@interface Month (CoreDataGeneratedAccessors)

- (void)addStatementsObject:(Statement *)value;
- (void)removeStatementsObject:(Statement *)value;
- (void)addStatements:(NSSet<Statement *> *)values;
- (void)removeStatements:(NSSet<Statement *> *)values;

@end

NS_ASSUME_NONNULL_END

和

#import "Statement.h"

NS_ASSUME_NONNULL_BEGIN

@interface Statement (CoreDataProperties)

@property (nullable, nonatomic, retain) NSNumber *amount;
@property (nullable, nonatomic, retain) NSNumber *isIncome;
@property (nullable, nonatomic, retain) NSString *title;
@property (nullable, nonatomic, retain) Month *month;

@end

NS_ASSUME_NONNULL_END

Answer 1

几乎正确，您必须创建两个实体，然后绑定(bind)它们的关系:

NSEntityDescription *month = [NSEntityDescription insertNewObjectForEntityForName:@"Month"  inManagedObjectContext:[self managedObjectContext]];
Statement *statement = [NSEntityDescription insertNewObjectForEntityForName:@"Statement" inManagedObjectContext:context];

statement.title = self.enterTextGreenViewTextField.text;

NSNumber  *aNum = [NSNumber numberWithDouble:savingAmount];
statement.amount =aNum;
statement.isIncome = [NSNumber numberWithBool:isIncome];

statement.month=month;

这一行我不知道是否需要手动生成集合，但我还是这样做了:

NSMutableSet *monthStatements;
if(month.statements){
     monthStatements=[NSMutableSet setWithSet:month.statements];
}else{
     monthStatements=[NSMutableSet alloc] init];
}
[monthStatements addObject:statement];
month.statements = monthStatements;

最后保存:

NSError *error=nil;
if (![context save:&error]) {
     FATAL_CORE_DATA_ERROR(error);
     return;
}

在我看来，这应该很有魅力(虽然还没有尝试过):-)

** 编辑:

获取:

    NSArray *fetchedObjects;
    NSError *error=nil;
    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"Store" inManagedObjectContext:[self managedObjectContext]];

    [fetchRequest setEntity: entity];

    NSPredicate *predicate = [NSPredicate predicateWithFormat: @"code==%@", code];

    [fetchRequest setPredicate:predicate];

    fetchedObjects = [[self managedObjectContext] executeFetchRequest:fetchRequest error:&error];

如果月份已经存在，您通常会尝试获取该月份，如果不存在，您可以使用我给您的代码创建它。我不知道你所说的建立新关系是什么意思......