ADO.NET EF Composite primary key - can not update foreign key

I have following entities:

In my code I need to update the FKCategoryID开发者_开发知识库 in entity BudgetPost but I'm getting the following error:

FKCategoryID is part of the object's key information

Is it any way to update the key in this case or it's not possible?

Thanks in advance

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Why is it a part of the composite key? As long as FKBudgetID is a part of the composite primary key you won't be able to modify it. If you want to enforce uniqueness in the combination of FKCategoryID and FKBudgetID, use a UNIQUE constraint instead.