NSManagedObjectContext+FetchAggregates.m

#import <CoreData/CoreData.h>

@implementation NSManagedObjectContext (FetchAggregates)

/**
 *  This method groups CoreData objects based on a list of user defined properties/relationships.
 *  Properties contain values, and Relationships are properties that specifically point to another NSManagedObject.
 *  Note: properties and relationships must be separated out as aggregate operations (count, max, min, avg) cannot be performed on relationships.
 *
 *  NOTE: A common use for this method is finding duplicate NSManagedObject entities where 'duplicate' means exactly matching certain properties/relationships.
 *
 *  This method finds all 'unique' groups of objects, where being unique means all objects in the group share the same values for the user defined properties/relationships.
 *  That is, if any two (or more) objects have the exact same value for each of the user defined properties/relationships 
 *   (ie obj1.field1 == obj2.field1 && obj1.fieldN == obj2.fieldN), those two (or more) objects will be grouped together (obj1 & obj2 make one group).
 *
 *  This method performs a count on the grouped objects and returns this count along with the property/relationship values that matched, making the group 'unique'.
 *  Example:
 *     Using a CarReservation entity, find any groups of CarReservations that have the same lastName for the same booked Car.
 *     This shows which Cars the user has decided to book more than once.
 *     Call the method with the input parameters:
 *                                                propertyNames     = @["lastName"]
 *                                                relationshipNames = @["car"]
 *                                                entityName        = @"CarReservation"
 *  The returned array could contain:
 *                                   @[  @{ "count"=2, "lastName"="Smith", "car"="<NSManagedObjectID:p1234>" },
 *                                       @{ "count"=1, "lastName"="Brown", "car"="<NSManagedObjectID:p4321>" },
 *                                       @{ "count"=1, "lastName"="Smith", "car"="<NSManagedObjectID:p4321>" },
 *                                       @{ "count"=1, "lastName"="Winehouse", "car"=nil } ]
 *
 *  This can be interpreted as the user having:
 *                                             2 reservations for car 'p1234' under name Smith (this car booked > once by Ms Smith)
 *                                             1 reservation for car 'p4321' under name Smith
 *                                             1 reservation for car 'p4321' under name Brown
 *                                             1 reservation under name Winehouse, but no car (this is an invalid CarReservation object!)
 *
 *
 *  @param propertyNames     NSArray of property name strings (any property of the entity EXCLUDING relationships) to group by. Must be non-empty.
 *  @param relationshipNames NSArray of relationship name strings (properties pointing to another NSManagedObject) to group by. Optional.
 *  @param entityName        Name of entity for which to count aggregate properties
 *  @param context           Context to fetch properties from
 *
 *  @return NSArray of dictionaries containing the count and property values shared for each object group
 */
 
- (NSArray *)countGroupedByProperties:(NSArray *)propertyNames
               groupedByRelationships:(NSArray *)relationshipNames
                            forEntity:(NSString *)entityName
{
    NSAssert(entityName.length, @"Cannout count aggregate properties if no entityName passed in");
    NSAssert(propertyNames.count, @"Cannot count aggregate properties if no propertyNames passed in");
    if (!propertyNames.count || !entityName.length)
    {
        return nil;
    }
    
    NSFetchRequest *fetchRequest = [NSFetchRequest fetchRequestWithEntityName:entityName];
    
    // Create the expression that gets the count of entities which are unique according to groupBy propertyNames
    NSString *expressionString = [NSString stringWithFormat:@"%@:(%@)", @"count", propertyNames.firstObject];
    NSExpression *countExpr = [NSExpression expressionWithFormat:expressionString];
    NSExpressionDescription *countExprDesc = [[NSExpressionDescription alloc] init];
    countExprDesc.name = @"count";
    countExprDesc.expression = countExpr;
    countExprDesc.expressionResultType = NSInteger64AttributeType;
    
    // For each propertyName, get the corresponding NSAttributeDescription
    NSMutableArray *desiredEntityProperties = [[NSMutableArray alloc] init];
    NSDictionary *allEntityProperties = [self.persistentStoreCoordinator.managedObjectModel.entitiesByName[entityName] propertiesByName];
    for (NSString *propertyName in propertyNames)
    {
        NSAttributeDescription *propertyAttribute = allEntityProperties[propertyName];
        NSAssert(propertyAttribute, @"propertyName '%@' not found on entity '%@'. Returning nil", propertyName, entityName);
        if (!propertyAttribute)
        {
            return nil;
        }
        [desiredEntityProperties addObject:propertyAttribute];
    }
    for (NSString *relationshipName in relationshipNames)
    {
        NSRelationshipDescription *relationshipAttribute = allEntityProperties[relationshipName];
        NSAssert(relationshipAttribute, @"relationshipName '%@' not found on entity '%@'. Returning nil", relationshipName, entityName);
        if (!relationshipAttribute)
        {
            return nil;
        }
        [desiredEntityProperties addObject:relationshipAttribute];
    }

    fetchRequest.propertiesToGroupBy = desiredEntityProperties;
    fetchRequest.propertiesToFetch   = [desiredEntityProperties arrayByAddingObject:countExprDesc];
    fetchRequest.resultType          = NSDictionaryResultType;
    
    NSError *error;
    NSArray *entityPropertyCounts = [self executeFetchRequest:fetchRequest error:&error];
    return entityPropertyCounts;
}

@end