Why is EF trying to insert NULL in id-column?

Question

I am writing my project using Entity Framework 4.0 (Model first). At the beginning of the project, I faced with this problem: I am trying to insert the filled object in the

Answer 1

Have a look at this: https://stackoverflow.com/a/5338384/171703 - entity framework might be assuming that your CategoryId field is an identity and therefore passing null to the database expecting it to fill it for you.