NSInMemoryStoreType is not a good substitute in unit tests

So I was writing some tests for my KidChart app, which uses core data, and I wasted a ton of time, so I thought I'd post to warn people.  I wanted to avoid having to reset the state for each test, and I wanted the tests to run quickly, so I used NSInMemoryStoreType for my persistent store in my unit tests. This is a technique I've used before in other programming languages, and I was new to Core Data, so I was applying what I had done before to something I had insufficiently researched.

The app has a pretty simple data model.  There are Events, and each Event has a Child and a Behavior, and each Behavior has a good/bad score attached to it.  On my home screen, there's a summary view, where I show the number of events for each child for each (good/bad) score for that day. I was thinking I could use an NSPredicate to grab just what I wanted.  Basically, what I wanted was:

1 SELECT Child.Name, Behavior.Score,count(Event.*)   
2 FROM Child, Behavior, Event
3 WHERE Child.event.id = Event.id AND
4 Event.Behavior_id = Behavior.id AND
5 Event.date > 'Today at Midnight'
6 GROUP BY Child.Name, Behavior.Score

Except you can't have direct SQL access.  So what I ended up with (after much iteration) was:

1 NSFetchRequest *request = [[NSFetchRequest alloc] init];
2 NSEntityDescription *entity = [NSEntityDescription entityForName:@"Child" 
3                               inManagedObjectContext:managedObjectContext];
4 [request setEntity:entity];
5 
6 // Specify that the request should return dictionaries.
7 [request setResultType:NSDictionaryResultType];
8 
9 // Create an expression for the key path.
10 NSExpression *childEventKeyPathExpression = [NSExpression expressionForKeyPath:
11                                             @"events"];
12 NSExpression *onlyHappyEvents = [NSExpression expressionForSubquery:
13                                 childEventKeyPathExpression 
14                         usingIteratorVariable:@"e" 
15                               predicate:[NSPredicate predicateWithFormat:@"$e.
16                               behavior.score == 1 and $e.timeStamp > %@" 
17                               argumentArray:[NSArray arrayWithObject: currentDate]]];
18 
19 // Create an expression to represent the count value at the key path 'events'
20 NSExpression *happyCountExpression = [NSExpression expressionForFunction:@"count:" 
21                                      arguments:[NSArray arrayWithObject:
22                                      onlyHappyEvents]];
23 
24 
25 // Create an expression description using the minExpression and returning a date.
26 NSExpressionDescription *happyCountExpressionDescription = [[NSExpressionDescription 
27                                                            alloc] init];
28 
29 // The name is the key that will be used in the dictionary for the return value.
30 [happyCountExpressionDescription setName:@"happyEventCount"];
31 [happyCountExpressionDescription setExpression:happyCountExpression];
32 [happyCountExpressionDescription setExpressionResultType:NSInteger16AttributeType];
33 
34 NSExpression *onlySadEvents = [NSExpression expressionForSubquery:
35                               childEventKeyPathExpression 
36                         usingIteratorVariable:@"e" 
37                               predicate:[NSPredicate predicateWithFormat:@"$e.
38                               behavior.score == -1 and $e.timeStamp > %@" 
39                               argumentArray:[NSArray arrayWithObject: currentDate]]];
40 // Create an expression to represent the count value at the key path 'events'
41 NSExpression *sadCountExpression = [NSExpression expressionForFunction:@"count:" 
42                                    arguments:[NSArray arrayWithObject:onlySadEvents]]
43                                    ;
44 
45 // Create an expression description using the minExpression and returning a date.
46 NSExpressionDescription *sadCountExpressionDescription = [[NSExpressionDescription 
47                                                          alloc] init];
48 
49 // The name is the key that will be used in the dictionary for the return value.
50 [sadCountExpressionDescription setName:@"sadEventCount"];
51 [sadCountExpressionDescription setExpression:sadCountExpression];
52 [sadCountExpressionDescription setExpressionResultType:NSInteger16AttributeType]; 
53 
54 NSExpression *childNameKeyPathExpression = [NSExpression expressionForKeyPath:
55                                            @"Name"];
56 NSExpressionDescription *nameExpressionDescription = [[NSExpressionDescription alloc]
57                                                      init];
58 [nameExpressionDescription setName:@"childName"];
59 [nameExpressionDescription setExpression:childNameKeyPathExpression];
60 [nameExpressionDescription setExpressionResultType:NSStringAttributeType];
61 
62 // Set the request's properties to fetch just the property represented by the 
63 expressions.
64 [request setPropertiesToFetch:[NSArray arrayWithObjects:nameExpressionDescription, 
65 happyCountExpressionDescription, sadCountExpressionDescription, nil]];
66   
67 // Execute the fetch.
68 NSArray *objects = [managedObjectContext executeFetchRequest:request error:&error];
69 STAssertNil(error, @"error not nil:%@",error);

Which is way more verbose than the corresponding SQL, but looks scarier than it actually is.  And it worked great.  And then, I changed my persistent store type to NSSQLiteStoreType.  And it started throwing "NSInvalidArgumentException: -predicate only defined for abstract class" errors. And I got to start all over.