Question

I new to iOS development and doing some test with the core data. It was all fine until I started playing with relationship.
In context:

I have 2 entity: Article and Category.

Article has two members idArticle and text. Articles has to have a single Category.
Category has two members idCategory and name. Category can have 0 or several Articles.

Article:

• idArticle
• text
• category (Relationship to Category, Inverse = articles, minimum optional, maximum 1)

Category:

• idCategory
• name
• articles (To many Relationship to Article, Inverse = category, minimum optional, maximum unlimited)

While I am adding an article. I am first surprise to not only have article1.category but also article1.idCategory and article1.name!
I have currently set all my attributes, relationship to optional.
Whatever I do, when I add a new article using the code below, it will add a new Category as well which will contain idCategory = 0 and name = nil if I dont set article.idCategory and article.name! or to the corresponding value if I set them. However, I don’t want it to create that category, I just want to add an existing category.
article.category works fine; it add the article to the right category! If I only set article.category; article.idCategory will = 0 and article.name = nil.

I suppose that I could delete the newly created category but I want my code to be neat. I have search the web but didnt find similar example with that problem. I am not dealing with any GUI here.
My code:

 - (BOOL)createNewGmtArticle:(NSNumber*)articleID title:(NSString*)paramTitle text:(NSString*)paramText date:(NSDate*)paramDate categoryID:(NSNumber*)paramCategoryID categoryName:(NSString*)paramCategoryName

{
    GasMattersTodayArticles *gmtArticle = [NSEntityDescription insertNewObjectForEntityForName:@"GasMattersTodayArticles" inManagedObjectContext:self.managedObjectContext]; // Look the given entitiy GasMattersTodayArticles in the given managed obj context

    if(gmtArticle != nil)
    {
        // Fill article
        gmtArticle.idArticle = articleID;
        gmtArticle.text = paramText;

        gmtArticle.category = [self getGmtCategoryWithId:paramCategoryID];
        //gmtArticle.idCategory = gmtArticle.category.idCategory; 
        //gmtArticle.name = gmtArticle.category.name;        

        NSError *savingError = nil;
        if([self.managedObjectContext save:&savingError]) // flush all unsaved data of the context to the persistent store
        {
            NSLog(@"Successfully saved the context");
            return YES;
        }
        else
        {
            NSLog(@"Failed to save the context. Error = %@", savingError);
            return NO;
        }
    }

    NSLog(@"Failed to create the new article");
    return NO;
}

and

-(GasMattersTodayCategory *)getGmtCategoryWithId:(NSNumber*)categoryID

{
    // Create the fetch request first
    NSDictionary *subs = [NSDictionary dictionaryWithObject:categoryID forKey:@"SEARCH_KEY"];    
    NSFetchRequest *fetchRequest = [self.managedObjectModel fetchRequestFromTemplateWithName:@"CategoryWithKey" substitutionVariables:subs];
    // Entity whose contents we want to read
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"GasMattersTodayCategory" inManagedObjectContext:self.managedObjectContext];

    // Tell the request that we want to read the content of the person entity
    [fetchRequest setEntity:entity];

    // Excecute the fetch request on the context
    NSError* requestError = nil;
    GasMattersTodayCategory *category = [[self.managedObjectContext executeFetchRequest:fetchRequest error:&requestError] lastObject];

    // Make sur we get a category
    if(category != nil)
    {
        return category;
    }
    else
    {
        NSLog(@"Could not find any Category entities with this Id in the context.");
        return nil;
    }    
}

Thanks! This super basic task is currently ruining my sunday!

Answer 1

The most likely way this could have happened is if your Article Entity is a sub-class of Category.

You would get exceptions otherwise when setting gmtArticle.idCategory telling you that GasMattersTodayArticles is not key-value compliant to idCategory.

GasMattersTodayArticles and GasMattersTodayCategory should both be sub-classes of NSManagedObject (or maybe a base class for your project).

You are creating a new category when you create an article because according to your structure an Article is-a Category.

However without seeing the .xcdatamodel this answer is a guess.