I had been using core data fetch request for a long time and while browsing about the fetch request, I came across a feature called Fetch Request Template.
This blog describes about the FetchRequestTemplate in detail.
What is Fetch Request Template :
Fetch request templates allow to pre-define queries and their parameters in the model (with the tool) – typically they contain variables that need to be substituted at runtime.
How It Works :
Let have a model with the entity student and attributed listed below :
Step 1 : Create the Template
Approach 1:
NSManagedObjectModel *model = // reference to model
NSFetchRequest *requestTemplate = [NSFetchRequest new];
NSEntityDescription *entityDescription = [[model entitiesByName] objectForKey:@“Student”];
[requestTemplate setEntity:entityDescription];
NSPredicate *predicateTemplate = [NSPredicate predicateWithFormat: @”(name like[cd] $NAME) AND (age > $AGE)”];
[requestTemplate setPredicate:predicateTemplate];
[model setFetchRequestTemplate:requestTemplate forName:@“StudentWithNameWithAge”];
Approach 2:
Go to your model (xcdatamodel) file, then Editor -> Add Fetch Request. The UI makes it easy to write a complex fetch request. Unlike the previous approach, you cannot store things like sort descriptors. You’d have to add those after you retrieve the template.
Step 2 : Access the Template
NSManagedObjectContext *moc = // reference to context
NSManagedObjectModel *model = // reference to model
NSError *error = nil;
NSDictionary *substitutionDictionary = [NSDictionary dictionaryWithObjectsAndKeys: myName, @“NAME”, age, @“AGE”, nil];
NSFetchRequest *fetchRequest = [model fetchRequestFromTemplateWithName:@“StudentWithNameWithAge” substitutionVariables:substitutionDictionary];
NSArray *results = [moc executeFetchRequest:fetchRequest error:&error];
// Process results
Points to Consider :
1If the template contains a predicate with substitution variables, you should instead use
fetchRequestFromTemplateWithName:substitutionVariables: to create a new fetch request.
2If you use fetchRequestFromTemplateWithName:substitutionVariables:. The variables dictionary must provide values for all the variables. If you want to test for a nil value, use [NSNull null]. This method provides the usual way to bind an “abstractly” defined fetch request template to a concrete fetch.
3If you use setFetchRequestTemplate:forName:. This method raises an exception if the receiver has been used by an object graph manager.
Happy Coding….