Problem with show item in ComboBox!

Sorry for my english!
I have a problem! I use the automatic generation of the form.
I set DataSource like type DocType for the form. DocType includes object type field DocGroup

public class DocType  {
        private int id;
        private String name;
        private String code;
        private String note;

	private DocGroup docGroup;
        //Getters and Setters...

This is DocGroup type.

public class DocGroup  {
        private int id;
        private String name;

        //Getters and Setters...

I’m using FormFieldFactory for generating Form’s fields. In the method createField I’m create ComboBox component.

	class MyFieldFactory implements FormFieldFactory
		private ComboBox select;

		public Field createField(Item item, Object propertyId,
				Component uiContext) {

			if (propertyId.equals("code")) {

				return new TextField("Code");

			} else if (propertyId.equals("name")) {

				return new TextField("Name");

			} else if (propertyId.equals("docGroup")) {

				select = new ComboBox("Doc group");

				select.addContainerProperty("docGroup", DocGroup.class, null);
                                JPAContainer<DocGroup> container = JPAContainerFactory.make(DocGroup.class, "Myunut");

	                        select.setValue(2); //DON'T WORK! WHY???
				return select;
			} else if (propertyId.equals("note")) {

				return new TextField("Note");

			return null;

When I’m change DocGroup field in form and commit the form, I’m received in beanItem null in all fields object DocGroup.

		BeanItem<DocType> beanItem = (BeanItem<DocType>) form.getItemDataSource();

How i can get changed DocGroup object or id of selected DocGroup item? And items don’t visible in ComboBox? Help me please!