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    <p>I'm not sure I understand your question, but at least I can answer how to get those objects that two Core Data managed objects have in common in a certain property.</p> <p>You have two Core Data entities, <code>A</code> and <code>C</code>. As all Core Data entities, they are represented by <code>NSManagedObject</code> instances at runtime. You have defined <code>A</code> and <code>C</code> to each have a relationship called <code>B</code>, which is one-to-many or many-to-many. In other words, each <code>A</code> object can have many <code>B</code> objects and so can <code>C</code>.</p> <p>To make this a bit easier I will come up with other names for these classes:</p> <pre><code>Dessert &lt;&lt;----&gt;&gt; Ingredient MainDish &lt;&lt;----&gt;&gt; Ingredient </code></pre> <p>Let's say the relationship is called <code>ingredients</code>. You can then get all ingredients for a dessert with</p> <pre><code>NSSet* dessertIngredients = [myDessert valueForKey:@"ingredients"]; </code></pre> <p>And likewise get the ingredients for the main dish like so: </p> <pre><code>NSSet* mainDishIngredients = [myMainDish valueForKey:@"ingredients"]; </code></pre> <p>To get the intersection of the two sets, you need to use a mutable set object:</p> <pre><code>NSMutableSet* commonIngredients = [NSMutableSet setWithSet:dessertIngredients]; [commonIngredients intersectSet:mainDishIngredients]; </code></pre> <p>Another way to get the same result is by using Cocoa's <a href="http://developer.apple.com/library/ios/#documentation/cocoa/conceptual/KeyValueCoding/Articles/CollectionOperators.html" rel="nofollow">KVC collection operator</a> <code>@distinctUnionOfArrays</code>:</p> <pre><code>NSArray* objectsWithIngredients = [NSArray arrayWithObjects:myMainDish, myDessert, anotherMainDish, nil]; NSSet* uniqueIngredients = [anotherMainDish valueForKeyPath:@"@distinctUnionOfArrays.ingredients"]; </code></pre>
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    1. POCore Data: three or more table joins
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    1. COWhy not use a predicate containing an 'AND' and let Core Data do the heavy lifting? Then there's no need to mess around with set intersections.
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    2. COYou're right. I only partially understood the question, and didn't think about using the inverse relationship anyway, which is clearly better.
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    3. COHey thanks for the reply, your approach is also good, and easy to understand, but I think predicate search with core data will be more efficient.
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