How to insert distinct records through core data in iPhone?

Question

I want to insert only those records in core data in iPhone which are not already present in the sqllite table. In other words, I want to insert distinct records in my core data table. My code of insertion is

for(NSInteger j=0;j<[items count];j++){
            Story *story=(Story *)[NSEntityDescription insertNewObjectForEntityForName:@"Story" inManagedObjectContext:managedObjectContext];

            [story setTitle:[[items objectAtIndex:j] objectForKey:@"title"]];
            [story setDate:[[items objectAtIndex:j] objectForKey:@"date"]];
     }

Tell me the way to insert only distinct records in this.

Answer 1

看一下这个页面 ，“有效地实现查找或创建”一节提供了有关如何实现更新/插入机制的详细信息。

Answer 2

One solution might be to give your entity an index attribute (just an integer value), and check for this before you add a new entity.

Alternatively, if you don't want duplicates to be possible, you could simply perform a fetch on stories with the same title and date . If none are returned from this fetch, then add the new object as in your own code. You can implement it like this:

NSString *title = [[items objectAtIndex:i] objectForKey:@"title"];
NSDate *date = [[items objectAtIndex:i] objectForKey:@"date"];

NSEntityDescription *entity = [NSEntityDescription entityForName:@"Story" inManagedObjectContext:managedObjectContext];

// Create the predicates to check for that title & date.
NSString *predTitleString = [NSString stringWithFormat:@"%@ == %%@", @"title"];
NSString *predDateString = [NSString stringWithFormat:@"%@ == %%@", @"date"];

NSPredicate *predTitle = [NSPredicate predicateWithFormat:predTitleString, @"title"];
NSPredicate *predDate = [NSPredicate predicateWithFormat:predDateString, @date];

NSArray *predArray = [NSArray arrayWithObjects:predTitle, predDate, nil];
NSPredicate *predicate = [NSCompoundPredicate andPredicateWithSubpredicates:predArray];

// Create the fetch request.
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
[fetchRequest setEntity:entity];
[fetchRequest setPredicate:predicate];

// Fetch results.
NSError *error = nil;
NSArray *array = [context executeFetchRequest:fetchRequest error:&error];

// If no objects returned, a story with this title & date does not yet exist in the model, so add it.
if ([array count] == 0) {
    Story *story=(Story *)[NSEntityDescription insertNewObjectForEntityForName:@"Story" inManagedObjectContext:managedObjectContext];

    [story setTitle:title];
    [story setDate:date];
}

[fetchRequest release];

I have found it very useful to implement a utility class that contains generic methods to perform these fetches, so all you have to do is tell it the name of your entity, a dictionary of keys & values to check for, and the context to search in. Saves rewriting a lot of code!

Hope this helps.