Quick Explanation of SUBQUERY in NSPredicate Expression

问题

There appears to be zero documentation about the SUBQUERY keyword from Apple and I can't find a simple explanation about it on SO or on Google. It's a conspiracy! ;)

Please, could someone from the inner-circle please just provide a quick explanation of its syntax so I can use it?

SUBQUERY(Bs, $x, $x IN %@)

Thanks

回答1:

This is what a subquery evaluates to. (Found from this mailing list thread, the #1 hit for “NSPredicate subquery” in Google.) That bit of documentation also explains how the predicate format string syntax relates to it.

回答2:

And for people who don't quite get what the documentation is saying, a SUBQUERY is essentially this:

SUBQUERY(collection, variableName, predicateFormat)

And could (simplistically) be implemented like this:

id resultingCollection = ...; //a new collection, either a mutable set or array
NSMutableDictionary * substitutions = [NSMutableDictionary dictionary];
NSPredicate * p = [NSPredicate predicateWithFormat:predicateFormat];
for (id variable in collection) {
  [substitutions setObject:variable forKey:variableName];
  NSPredicate * filter = [p predicateWithSubstitutionVariables:substitutions];
  if ([filter evaluateWithObject:collection] == YES) {
    [resultingCollection addObject:variable];
  }
}
return resultingCollection;

So in a nutshell, a SUBQUERY is basically taking a collection of objects and filtering out various objects based on the predicate expression of the SUBQUERY, and returning the resulting collection. (And the predicate itself can contain other SUBQUERYs)

Example:

NSArray * arrayOfArrays = [NSArray arrayWithObjects:
                           [NSArray arrayWithObjects:....],
                           [NSArray arrayWithObjects:....],
                           [NSArray arrayWithObjects:....],
                           [NSArray arrayWithObjects:....],
                           [NSArray arrayWithObjects:....],
                           [NSArray arrayWithObjects:....],
                           nil];
NSPredicate * filter = [NSPredicate predicateWithFormat:@"SUBQUERY(SELF, $a, $a.@count > 42)"];
NSArray * filtered = [arrayOfArrays filteredArrayUsingPredicate:filter];
//"filtered" is an array of arrays
//the only arrays in "filtered" will have at least 42 elements each

回答3:

Subquery represents a predicate (third argument - $x IN %@) that is evaluated on all objects (second argument - $x - it's like a variable name in foreach) of a relationship (first argument - Bs). Similarly to regular query returns a list of objects.

I see in many places that people use $x almost dogmatically, but $object in objects relationship makes perfect sense as well (or $city in cities...) :)

来源：https://stackoverflow.com/questions/3810992/quick-explanation-of-subquery-in-nspredicate-expression

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