I’m writing code that walks through an NSPredicate and generates an alternate
form of query from it (similar to what CoreData and Realm do.)
I’m seeing some very weird undocumented behavior when format strings are
compiled into NSPredicates, if the format string includes a key-path with a
property named “first” or “last”. The resulting predicate contains an
NSExpression of an undocumented type.
- For example, “name.middle == ‘Bob’” compiles to what you’d expect; the LHS of
the ‘equals’ predicate is an expression of NSKeyPathExpressionType, with
key-path “name.middle”.
- But “name.first == ‘Bob’” results in an LHS that’s of
NSFunctionExpressionType, with a function selector “valueForKeyPath:”, and even
weirder, the expression’s argument is an expression with the undocumented
expressionType 11. I don’t know what that is supposed to mean, except that its
-description is “FIRST” (all caps).
NSComparisonPredicate
predicateOperatorType = NSEqualToPredicateOperatorType
leftExpression =
NSKeyPathExpression
expressionType = NSFunctionExpressionType
function = “valueForKeyPath:”
operand =
NSKeyPathExpression
expressionType = NSKeyPathExpressionType
keyPath = “name”
operand = SELF
arguments = [
NSSymbolicExpression
expressionType = 11 (?????)
]
rightExpression =
NSExpression
expressionType = NSConstantValueExpressionType
constantValue = @“Bob”
Is there something magic about the property name “first”? The only related
thing I’ve seen is that in predicate syntax arrays can be indexed with
“[FIRST]” or “[LAST]”, but that’s in an array subscript not a keypath…
—Jens
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