I think you'll find everything you need in the ezobjectrelationtype.php file which controls how ezpublish manage object relation via datatype, especially in the storeObjectAttribute() and fetchObjectAttributeHTTPInput() functions. You can also have a look inside the ezobjectrelationlist If you need multiple relation functionnality. With a step by step debugger you'll be able to understand and reproduce what you need.
Hope this helps.
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Thanks for your answear. I have, as I wrote in my first post, tried to the delete-functions in that file. But I will take a look in thoose two functions you told me about.
If anyone else have some input on this topic. I would be very happy to see what they have on their mind :)