Using NSPredicate and Joins - ios

Question

I have 2 tables: Users and Comments like so:
Users
user_id (PK)
name
Comments
comment_id (PK)
user_id
comment
I want to get a list of all users and their comments.
How is this done using NSPredicate?
Thanks.

Answer 1

As far as I understood your question, you are using core-data.
First of all, you have to make a small change in your managed object model.
Entity
Users
Attributes
user_id
name
Entity
Comments
Attributes
comment_id
comment
Relationships
Relationship - - - Desination - - - Inverse
user - - - Users - - - NO Inverse
Now , whenever you enter the data for Comments object, associate a user relationship with it as
comment.user=theUser
where theUser is a 'Users' object which you can get from 'Users' table by comparing user_id, for that you can use a simple method as by calling the following method for the userID that you got from comment data
theUser= [self isUserAlreadyExistsWithUserID:userID];
where the method definition could be like in Users NSManagedObject subclass
+(NSManagedObject*)isUserAlreadyExistsWithUserID uniqueValue: (id)uniqueValue
{
NSManagedObjectContext* context=[self managedObjectContext];
NSError*error=nil;
NSEntityDescription *entity = [NSEntityDescription entityForName:#"Users" inManagedObjectContext:context];
NSPredicate *predicate = [NSPredicate predicateWithFormat:#"user_id = %#",uniqueValue];
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
fetchRequest.entity = entity;
[fetchRequest setPredicate:predicate];
NSArray *objects = [context executeFetchRequest:fetchRequest error:&error];
if(objects && objects.count>0)
{
return [objects objectAtIndex:0];
}
else
{
return nil;
}
}
Now, to fetch comments for particular user in comments NSManagedObject subClass:
+(NSFetchedResultsController*)getCommentsForUser:(Users*)user
{
NSString* cacheName=#"Root";
[NSFetchedResultsController deleteCacheWithName:cacheName];
NSManagedObjectContext* context=[self managedObjectContext];
NSEntityDescription *entity = [NSEntityDescription entityForName:NSStringFromClass([self class]) inManagedObjectContext:context];
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
[fetchRequest setEntity:entity];
NSPredicate* predicate=[NSPredicate predicateWithFormat:#"user=%#",user];
fetchRequest setPredicate:predicate];
NSFetchedResultsController *theFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest
managedObjectContext:context sectionNameKeyPath:nil
cacheName:cacheName];
return theFetchedResultsController;
}
Hope this helps.

Answer 2

I think, you need this query:
SELECT Users.*,Comments.comment_id,Comments.comment FROM Users LEFT JOIN Comments ON Users.user_id = Comments.user_id;
This will return list of all users along with their comments. You can modify it, as per your specific requirements.

Answer 3

Are you using CoreData? if yes then use try the following code:
NSEntityDescription *entityDescription = [NSEntityDescription entityForName:#"Users" inManagedObjectContext:self.managedObjectContext];
NSFetchRequest *request = [[NSFetchRequest alloc] init];
[request setEntity:entityDescription];
[request setRelationshipKeyPathsForPrefetching:[NSArray arrayWithObjects:#"Comments",nil]];
[request setIncludesSubentities:YES];
NSArray* returnArray = [self.managedObjectContext executeFetchRequest:request error:&error];
if([returnArray count] > 0)
{
Users* user = [returnArray objectAtIndex:0];
NSLog(#"%# %# %#", user.user_id, user.name, user.userComments.comment);
}
Also, CoreData uses relationships instead of foreignKeys. Therefore, to get this working you need to setup an inverse relationship userComments (many to one) from Comments to Users.

Answer 4

It is better not to use NSPredicate when making a query. You can simply write a JOIN query. As Apple's documentation also states that the NSPredicate class is used to define logical conditions used to constrain a search either for a fetch or for in-memory filtering. Also it will be easier to use Key-Value Coding. Here is a good explaination to how it can be used. Lets assume you have NSSet of Comments. You can do something like :
[Comments valueForKeyPath:#"users.user_id"];
But, if you really want to use NSPredicate following is the possible way:
NSFetchRequest *fetchReq = [[NSFetchRequest alloc] initWithEntityName:#"Users"];
NSError *error;
NSPredicate *predicate = [NSPredicate predicateWithFormat:#"(user_id == %#)",Comments.user_id];
[fetchReq setPredicate:predicate];
NSArray *result = [self.managedObjectContext executeFetchRequest:fetchReq error:&error];