Question

Consider I have 2 entities – a) Publisher b) Book
Publisher has navigation property called as PublishedBooks which is collection of Books.
Assume that Publisher1 has published 2 books ie Book1 & Book2
What I would like to do is, for a Publisher1, delete published book Book1 and
add a new published book (ie Book3), in the database.

Context.SaveChanges() throwing below error –

The operation failed: The relationship could not be changed because
one or more of the foreign-key properties is non-nullable. When a
change is made to a relationship, the related foreign-key property is
set to a null value. If the foreign-key does not support null values,
a new relationship must be defined, the foreign-key property must be
assigned another non-null value, or the unrelated object must be
deleted.

Note : Delete cascade rule is present in database as well as in the context class.
BTW, I am using C# & Sql Server 2005.

Answer 1

Sounds like you are doing something like this:

Foo foo = entity.RelatedFoos.Where(f=>f.id=xyz);
entity.RelatedFoos.Remove(foo); //this is the problematic line
context.SaveChanges();

That is; you are removing an entity from a set of related items – not from the main collection on the context itself. As noted in the error, removing the above way only removes the relation. Not the object foo. This is probably what you meant, if you mean to delete the foo:

Foo foo = entity.RelatedFoos.Where(f=>f.id=xyz);
context.Foos.Remove(foo);
context.SaveChanges();