Getting data to save with foreign key

A parent-child relation in the same table must be NULL-able.
So, set your parent column to allow NULL.

Or if you wish to keep it at NOT NULL, you have to create a menu item with ID 0, that will never be shown in the web, and set your parent column to default value 0. Although the first method would be prefered I suppose.

Edit: why nullable? because the top level item can not have a parent.

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