Help me to get the result based on condition

I have created a Users class based on NSManagedObject with following attributes (id,name,age etc).

I am using the core data model but i am not sure how to do the follwing...

Now i would like to know How c开发者_开发百科an i get the user detail based on user id.

example: select * from users where id = 1

please help me out.

You should use NSPredicate class for executing SQL commands. The code:

NSManagedObjectContext *context = self.managedObjectContext; // specify your MOC object

NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"users" inManagedObjectContext:context]; // specify your entity (table)
NSPredicate *predicate = [NSPredicate predicatewithFormat:@"id == %d",yourID]; // specify your condition (predicate)

[fetchRequest setEntity:entity];
[fetchRequest setPredicate:predicate];

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

[entity release];
[predicate release];
[fetchRequest release];

if (array == nil) {
    // Error: no objects returned
} else {
    // Success: do whatever you want
}

Step 1: Alloc/init NSFetchRequest

You need to alloc/init a NSFetchRequest object if you want to execute queries.

Step 2: Select entity

If you want to specify select * from users ..., you should use NSEntityDescription:

NSEntityDescription *entity = [NSEntityDescription entityForName:@"users" inManagedObjectContext:context];

At the end you need to 'attach' your entity description to your NSFetchRequest object via:

[fetchRequest setEntity:entity];

Step 3: Condition

If you want to have a condition (e.g. ... where id = 1), you have to implement NSPredicate.

NSPredicate *predicate = [NSPredicate predicatewithFormat:@"id == %d",yourID];

yourID must be a number (e.g. 1, 2, 7 or 46).

And, again:

[fetchRequest setPredicate:predicate];

Step 4: Let's execute it!

NSArray *array = [context executeFetchRequest:fetchRequest error:&error];

All the records that meet the conditions will be returned as array of NSManagedObjects.

Step 5: Release objects

[entity release];
[predicate release];
[fetchRequest release];

Step 6: Do something

If there are no objects that meet the conditions, array object will be nil. You can check it and deal with the error via:

if (array == nil)

Check out Core Data Programming Guide for more info. :)