ios - Searching/Filtering a custom class array with a NSPredicate

Question

I have a array that contains objects of a custom class, and I would like to filter the array based on if one of the classes attributes contains a custom string. I have a method that is passed the attribute that I want to be searched (column) and the string that it will search for (searchString). Here is the code I have:

NSPredicate *query = [NSPredicate predicateWithFormat:@"%K contains %K", column,   searchString];
NSMutableArray *temp = [displayProviders mutableCopy];
[displayProviders release];
displayProviders = [[temp filteredArrayUsingPredicate:query] mutableCopy];
[temp release];

However, it always throws an exception on displayProviders = [[temp filteredArrayUsingPredicate:query] mutableCopy]; saying this class is not key value coding-compliant for the key [whatever searchString is].

Any ideas what I am doing wrong?

Answer 1

[NSPredicate predicateWithFormat:@"%@ contains %@", column,      searchString];

When you use the %@ substitution in a predicate format string, the resulting expression will be a constant value. It sounds like you don't want a constant value; rather, you want the name of an attribute to be interpreted as a key path.

In other words, if you're doing this:

NSString *column = @"name";
NSString *searchString = @"Dave";
NSPredicate *p = [NSPredicate predicateWithFormat:@"%@ contains %@", column, searchString];

That is going to be the equivalent to:

p = [NSPredicate predicateWithFormat:@"'name' contains 'Dave'"];

Which is the same as:

BOOL contains = [@"name rangeOfString:@"Dave"].location != NSNotFound;
// "contains" will ALWAYS be false
// since the string "name" does not contain "Dave"

That's clearly not what you want. You want the equivalent of this:

p = [NSPredicate predicateWithFormat:@"name contains 'Dave'"];

In order to get this, you can't use %@ as the format specifier. You have to use %K instead. %K is a specifier unique to predicate format strings, and it means that the substituted string should be interpreted as a key path (i.e., name of a property), and not as a literal string.

So your code should instead be:

NSPredicate *query = [NSPredicate predicateWithFormat:@"%K contains %@", column, searchString];

---

Using @"%K contains %K" doesn't work either, because that's the same as:

[NSPredicate predicateWithFormat:@"name contains Dave"]

Which is the same as:

BOOL contains = [[object name] rangeOfString:[object Dave]].location != NSNotFound;