Core Data does not save one-to-many relationship

Question

I have two entities, Document and Attachment. A document can have many attachments. An attachment can belong to a single document.

For some reason, Core Data is not saving the relationship between a document and its attachments when I do the following:

[document addAttachments:attachments];
[context save:&error];
[context reset];

If I fetch the document after executing the above code, its attachments will be empty. Yet, the attachments are saved.

The relationship is saved if I set the inverse relationship manually:

[document addAttachments:attachments];
for (Attachment *attachment in attachment)
{
    attachment.document = document;
}
[context save:&error];
[context reset];

I thought it wasn't necessary to set a relationship both ways with Core Data. What am I doing wrong?

In case it helps, the relationships are defined as follows:

attachments

• Transient: NO
• Optional: YES
• Destination: Attachment
• Inverse: document
• Delete Rule: Cascade
• Type: To Many
• Ordered: NO

document

• Transient: NO
• Optional: YES
• Destination: Document
• Inverse: attachments
• Delete Rule: Nullify
• Type: To One

Also tried

• Using mutableSetValueForKey: instead of the generated accessor.
• Adding the attachments one by one.
• Retaining the attachments array, just in case.

Same result.

Answer 1

This should work. Maybe these two answers might help Core Data Inverse Relationship Not Being Set

How to get at a relationship items properties in Core Data?

Can you post some code?