NSPredicate在一对多关系上引用多个字段？ [英] NSPredicate that references multiple fields on a to-many relationship?

问题描述

我使用的是SQLite永久存储。我有一个 NSManagedObject 类 Den 与一对多关系 Bear 。 Bear 有多个栏位：

I'm using a SQLite persistent store. I have a NSManagedObject class Den with a to-many relationship Bear. Bear has several fields:

Bear:
    breed
    color
    age
    ...

请求我的 Den 对象，我可以筛选具有相关 Bear 具有某个字段值的对象：

When I am building fetch requests for my Den objects, I can filter to objects that have a related Bear with a certain field value:

NSPredicate *hasGrizzlyPred = [NSPredicate predicateWithFormat:@"ANY Bear.breed == 'grizzly'"];

或者我也可以很容易地搜索 c>有一个棕色的熊：

Or I can just as easily search for a Den that has a brown bear:

NSPredicate *hasBrownBearPred = [NSPredicate predicateWithFormat:@"ANY Bear.color == 'brown'"];

但是有任何方法可以搜索 c>有一个熊是棕色和灰熊的？以下是合法的，但不正确，我认为：

But is there any way to search for a Den that has a bear that is both brown and a grizzly? The following is legal, but incorrect, I think:

// Not quite right: search for a den with a brown bear AND a grizzly
NSPredicate *hasBrownAndGrizzlyPred = [NSPredicate predicateWithFormat:@"ANY Bear.color == 'brown' AND ANY Bear.breed == 'grizzly'"];

推荐答案

http://developer.apple.com/mac/library/documentation/Cocoa/Reference/Foundation/Classes/NSExpression_Class/Reference/NSExpression.html#//apple_ref/doc/uid/TP30001190-SW22rel =nofollow noreferrer > SUBQUERY 谓词表达式。在对熊的查询的情况下（其中 Den 与 Bear 具有一对多的关系， code> bears ）是棕色的和灰熊的：

You can do this with a SUBQUERY predicate expression. In the case of a query for dens with a bear (where Den has a to-many relationship to Bear named bears) that is both brown and a grizzly:

[NSPredicate predicateWithFormat：@SUBQUERY （bears，$ b，$ b.color =='brown'AND $ b.breed =='grizzly'）。@ count> 0];

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