Help inserting many-to-many relatioship

Hi,

I'm painfully learning EOF (are there any good docs available??) and 
I'm stuck trying to insert a new Story record that is many-to-many 
related to Category records.

I've tried a few things, and the most correct seeming thing I was able 
to come up with was this, which is basically to go through each 
category that a story will be related to and 
addObjectToBothSidesOfRelationshipWithKey.

 >>>>
story.takeValueForKey(((Session)session()).getUserId(), "userId");
story.takeValueForKey(new NSTimestamp(), "datePosted");
ec.insertObject(story);

//for each category, add the story to both sides of the relationship
Enumeration		ite = selectedCategories.objectEnumerator();
while (ite.hasMoreElements())
{
	EOGenericRecord cat = (EOGenericRecord)ite.nextElement();
	cat.addObjectToBothSidesOfRelationshipWithKey(story, "stories");
}

ec.saveChanges();

//refault the object so all relationships are loaded next fetch
ec.invalidateObjectsWithGlobalIDs(new 
NSArray(ec.globalIDForObject(story)));
<<<<

However, I get a baffling exception,

EvaluateExpression failed: <snip>: "INSERT INTO story(user_id, 
date_posted, url, story_id, department, category_id, title, content, 
comment_id) VALUES (?, ?, NULL, NULL, ?, NULL, ?, ?, NULL)" 
withBindings: 1:1(userId), 2:2002-10-29 16:29:53(datePosted), <snip>

Why is EOF trying to insert a story record with NULL values? It has the 
correct bindings (as withBindings: shows). If I take out that loop in 
that code snippet, it is able to insert properly. Is there something I 
have to do properly in my EOModel?

Thanks for any info,

Sarwat.