Question

I’m trying to use nspredicate to find a managed object whose attribue’s vale is == to a value I want to check.

I know for certain that the value is in my data model.

This is what I have tried so far but the results array count is always zero.

-(BOOL)checkIfFavouriteExists:(NSString *)data
{
    BOOL returnvalue;

    NSFetchRequest *fetchReq = [[NSFetchRequest alloc]init];

    NSString *attributeName = @"userid";
    NSString *attributeValue = data;
    NSPredicate *predicate = [NSPredicate predicateWithFormat:@"%K == '%@'",
                              attributeName, attributeValue];

    //setting the predicate to the fetch request 
    [fetchReq setPredicate:predicate];

    [fetchReq setEntity:[NSEntityDescription entityForName:@"Favourites" inManagedObjectContext:self.managedObjectContext]];

    NSMutableArray *resultArray = [[NSMutableArray alloc]initWithArray:[self.managedObjectContext executeFetchRequest:fetchReq error:nil]];

    if([resultArray count]>0)
    {
        returnvalue=YES;

    }    else
    {
        NSLog(@"No records");
        returnvalue=NO;
    } 

    return returnvalue;
}

EDIT

The problem seems to be with my query format.

This works though?

NSExpression *exprName = [NSExpression expressionForKeyPath:@"userid"];
NSExpression *exprVal = [NSExpression expressionForConstantValue:data];
NSPredicate *predicate = [NSComparisonPredicate predicateWithLeftExpression:exprName
                                                            rightExpression:exprVal 
                                                                   modifier:NSDirectPredicateModifier 
                                                                       type:NSEqualToPredicateOperatorType 
                                                                    options:0];

I dont understand why one works and the other does not. They both do the same thing right?

Answer 1

Don’t put your variables in quotes. The documentation on Parser Basics states:

Single or double quoting variables (or
substitution variable strings) cause
%@, %K, or $variable to be interpreted
as a literal in the format string and
so prevent any substitution.

This should do it:

NSPredicate *predicate = [NSPredicate predicateWithFormat:@"userid == %@", attributeValue];

You can read more about predicates in the Predicate Programming Guide.