Fetched Property in XCode Data Model Editor for minimum value

Question

How do I add a Fetched Property in XCode's Data Model Editor for minimum value of one attribute??

My model:

Item (name, note, storedItem)
StoredItem (price, item)
Item 1 ---> N StoredITem (1->N Relationship)

I want that Item has a fetched property named minPrice and its value is the minimum value setted for price in the storedItems.

Example:

Item1 (banana, storedItem1 ... storedItem4, 10)
StoredItem1 (10,item1)
StoredItem2 (15,item1)
StoredItem3 (30,item1)
StoredItem4 (54,item1)

What do I put in Destination?? StoredItem?

And what do I put in Expression??