Why do I need to Insert and Update methods to my DomainService?

If I don't add Insert and Update methods to my domain service, I get exceptions when I try to add entities to the associated EntityCollection of my Entity. Now that I've added them (completely blank) I can add entities and modify them on the client but they never show up in my database. What am I missing? Do I need to create my own Insert and Update methods to my domain service and if so, what on earth would I put in them?

Edits:

This is what I have in my DomainContext. This seems a bit superfluous; I would think the Entity Framework would already do this.

    [Update]
    public void UpdateProject(Project a_project)
    {
      开发者_运维百科  ObjectContext.AcceptAllChanges();
    }

    [Update]
    public void UpdateProjectItem(ProjectItem a_projectItem)
    {
        ObjectContext.AcceptAllChanges();
    }

    [Insert]
    public void InsertProjectItem(ProjectItem a_projectItem)
    {
        ObjectContext.ProjectItems.AddObject(a_projectItem);
        ObjectContext.AcceptAllChanges();
    }

And this is how I'm using this on the client.

    ProjectItem projectItem = new ProjectItem();
    _reservedProject.Status = Project.ProjectStatusSubmitted;
    _reservedProject.ProjectItems.Add(projectItem);
    projectItem.LibraryItem = a_item;

    _projectItems.Add(projectItem);

    _domainContext.SubmitChanges();

UpdateProjectItem is never called.