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Aprove answer 
RKManagedObjectMapping* clientMapping = [RKManagedObjectMapping mappingForClass: [Client class]];
clientMapping.primaryKeyAttribute = @"clientID";
[clientMapping mapKeyPathsToAttributes:
@"id", @"clientID",
nil];

RKManagedObjectMapping* userFileMapping = [RKManagedObjectMapping mappingForClass:[UserFile class]];
userFileMapping.primaryKeyAttribute = @"userFileID";
[userFileMapping mapKeyPathsToAttributes:
 @"id", @"userFileID",
 @"client_id", @"clientID",
 nil];

[userFileMapping mapRelationship:@"client" withObjectMapping:clientMapping];

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