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POFiltering many-to-many count expression using existing subquery
 

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  1. POFiltering many-to-many count expression using existing subquery
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    <p>In my app, I have a many-to-many relationship between tags and links as follows :</p> <pre><code>Tags &lt;&lt;--&gt;&gt; Links </code></pre> <p>I am trying to return a list of the tags that relate to links that have the currently active tags, but are not included in the active tags. </p> <p>I also want to obtain a count of the number of links that have the 'other' tags, which needs to be limited by the active tags.</p> <p>Using the below, I have been able to return the 'other' tags and a count of links, but the count returned is of all links for each tag.</p> <p>I would like to be able to count the links using a similar approach to the one I'm using to build the subquery, but am struggling to get it to work. I have tried using the subquery generated in the count NSExpression, but this errors when the subquery is evaluated.</p> <pre><code>// Test array of tag names self.activeTagArray = [@[@"tag1", @"tag2"] mutableCopy]; NSFetchRequest *fetchRequest = [NSFetchRequest fetchRequestWithEntityName:[Tag entityName]]; // We want to exclude the tags that are already active NSPredicate *activeTagsPredicate = [NSPredicate predicateWithFormat:@"NOT ANY name IN %@", self.activeTagArray]; // Build subquery string to identify links that have all of the active tags in their tag set NSString __block *subquery = @"SUBQUERY(links, $link, "; [self.activeTagArray enumerateObjectsUsingBlock:^(id tagName, NSUInteger index, BOOL *stop) { if (index == self.activeTagArray.count - 1) { subquery = [subquery stringByAppendingString:[NSString stringWithFormat:@"SUBQUERY($link.tags, $tag, $tag.name = '%@') != NULL", tagName]]; } else { subquery = [subquery stringByAppendingString:[NSString stringWithFormat:@"SUBQUERY($link.tags, $tag, $tag.name = '%@') != NULL AND ", tagName]]; } }]; subquery = [subquery stringByAppendingString:@") != NULL"]; NSLog(@"Subquery : %@", subquery); NSPredicate *noTagsPredicate = [NSPredicate predicateWithFormat:subquery]; // Create a predicate array NSArray *predicateArray = @[noTagsPredicate, activeTagsPredicate, userPredicate]; NSPredicate *compoundPredicate = [NSCompoundPredicate andPredicateWithSubpredicates:predicateArray]; fetchRequest.predicate = compoundPredicate; fetchRequest.relationshipKeyPathsForPrefetching = @[@"links"]; // Set up the count expression NSExpression *countExpression = [NSExpression expressionForFunction: @"count:" arguments:@[[NSExpression expressionForKeyPath: @"links.href"]]]; NSExpressionDescription *expressionDescription = [[NSExpressionDescription alloc] init]; expressionDescription.name = @"counter"; expressionDescription.expression = countExpression; expressionDescription.expressionResultType = NSInteger32AttributeType; fetchRequest.propertiesToFetch = @[@"name", expressionDescription]; // Sort by the tag name NSSortDescriptor *sortDescriptor = [NSSortDescriptor sortDescriptorWithKey:@"name" ascending:YES]; fetchRequest.sortDescriptors = @[sortDescriptor]; fetchRequest.resultType = NSDictionaryResultType; NSError *error = nil; NSArray *resultsArray = [self.managedObjectContext executeFetchRequest:fetchRequest error:&amp;error]; if (error) { NSLog(@"Error : %@", [error localizedDescription]); } NSMutableArray *allTags = [[NSMutableArray alloc] init]; for (NSDictionary *tagDict in resultsArray) { NSLog(@"Tag name : %@, Link Count : %@", tagDict[@"name"], tagDict[@"counter"]); [allTags addObject:tagDict[@"name"]]; } [allTags addObjectsFromArray:self.activeTagArray]; </code></pre> <p>Any help with this would be greatly appreciated!</p>
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    1. USrobotpukeko
    1. USrobotpukeko
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    1. COI don't yet understand what exactly you want to count (and what is `links.href`?) An example would be helpful. - But note that you can simplify the `noTagsPredicate` to `SUBQUERY(links, $link, ANY $link.name in %@).@count > 0), self.activeTagArray`, making the enumerateObjectsUsingBlock loop obsolete.
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      1. USMartin R
    2. COThe loop builds a set of nested subqueries that determines if each of the tags is associated with that link, so the predicate replacement suggested won't do the same thing. The issue is that I am going from Tags, where I identify the active tags I am interested in, to determine the Links they associate with. From there, I go back to Tags from the Links identified to find any other remaining tags for those links that are not in the active set.
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      1. USrobotpukeko
    3. COYou are right, it is not the same predicate, my fault. (I still think that it can be simplified, but that is not your question.) - So for each fetched (other) tag, you want the count of links of that tag which are related to all given active tags, is that correct?
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      1. USMartin R
 

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