使用 NSPredicate 和 Core Data 建立深层关系

Question

我有一个 NSArrayController companiesController 绑定到顶级核心数据实体 Companies。

一个Company 有很多Department，一个Department 有很多Employee；这些由一对多关系表示，departments 和 employees。

基于Employee 的salary 属性，我认为我可以动态地执行此操作，以便在调用UI 的方法中根据薪水进行过滤：

NSPredicate *predicate = [NSPredicate predicateWithFormat:@"ANY departments.employees.salary < %@", [NSNumber numberWithInt:23000]];
[companiesController setFilterPredicate:predicate];

唉，这给了我错误：-[NSCFSet compare:]: unrecognized selector sent to instance。

Answer 1

在这种情况下不允许使用多对多键。

相反，您可以执行以下操作：

1. 通过将“过滤器”标志（布尔）属性添加到部门实体来修改数据模型。
2. 创建一个方法：获取所有部门对象，将满足谓词后半部分条件的部门的过滤标志设置为 YES，将其他部门的过滤标志设置为 NO，然后保存。
3. 在公司谓词中使用过滤器标志。

代码更改（第 3 步）：

    //NSPredicate *predicate = [NSPredicate predicateWithFormat:@"ANY departments.employees.salary < %@", [NSNumber numberWithInt:23000]];
    [self setDeptFilter:23000];
    NSPredicate *predicate = [NSPredicate predicateWithFormat:@"ANY depts.filter == YES"];
    [companiesController setFilterPredicate:predicate];

以及新方法（步骤2）：

- (void)setDeptFilter:(NSUInteger)salary {
    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];

    NSEntityDescription *entity = [NSEntityDescription entityForName:@"Department" inManagedObjectContext:self.managedObjectContext];
    [fetchRequest setEntity:entity];

    NSError *error = nil;

    // fetch all Department objects
    NSArray *array = [self.managedObjectContext executeFetchRequest:fetchRequest error:&error];

    [fetchRequest release];

    if (error) {
        NSLog(@"Error fetching Departments %@, %@", error, [error userInfo]);
        abort();
    }

    NSPredicate *predicate = [NSPredicate predicateWithFormat:@"ANY emps.salary < %@",[NSNumber numberWithInteger:salary]];
    NSArray *filterArray = [array filteredArrayUsingPredicate:predicate];

    // set filter flag to YES for the departments that meet the criteria
    for (Department *dep in filterArray) {
        dep.filter = [NSNumber numberWithBool:YES];
    }

    NSMutableArray *diffArray = [array mutableCopy];
    [diffArray removeObjectsInArray:filterArray];

    // set filter flag to NO for the departments that do NOT meet the criteria
    for (Department *dep in diffArray) {
        dep.filter = [NSNumber numberWithBool:NO];
    }

    [diffArray release];

    // save
    if ([self.managedObjectContext hasChanges] && ![self.managedObjectContext save:&error]) {
        NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
        abort();
    } 
}

Answer 2

您也可以使用子查询来做到这一点。

获取所有部门。 “of”关系是公司与多部门的逆关系：

-(void)printDepartmentsWithSalaryHigherThan:(int)salary inContext:(NSManagedObjectContext *)context {    
    NSFetchRequest *request = [[NSFetchRequest alloc ]init];
    request.entity = [NSEntityDescription entityForName:@"Department" inManagedObjectContext:context];
    request.predicate = [NSPredicate predicateWithFormat:@"SUBQUERY(employees, $emp, $emp.salary > %@ ).@count > 0", [NSNumber numberWithInt:salary]];

    for(Department *dep in [context executeFetchRequest:request error:nil]){
        NSLog(@"Department: %@", dep.depName);
        NSLog(@"in Company: %@", dep.of.compName);
    }
    [request release];
}

或者，如果您有更多公司，并且只希望公司的员工薪水“高于”某个金额。基于子查询结果的子查询

-(void)printCompaniesWithHigherSalaryThan:(int)salary inContext:(NSManagedObjectContext *)context {
    NSFetchRequest *request = [[NSFetchRequest alloc ]init];
    request.entity = [NSEntityDescription entityForName:@"Company" inManagedObjectContext:context];
    request.predicate = [NSPredicate predicateWithFormat:@"SUBQUERY(departments, $dep, SUBQUERY($dep.employees,$emp,$emp.salary > %@).@count > 0 ).@count > 0", [NSNumber numberWithInt:salary]];

    for(Company *c in [context executeFetchRequest:request error:nil]){
        NSLog(@"Company: %@", c.compName);
    }
    [request release];
}