>>>My UI allows user to enter the data; so there is a potential for them to enter wrong value; nothing I can about it. If I give them only combo boxes for selecting from pull-down lists some won't be happy. Thank you for your input.
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>>When user is creating a child record, he/she should not be required to type the parent key. The PK is inserted automatically with the child record.
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>Did I say PK anywhere in this discussion?

This is what you said: "For example, in my app user has to enter, say, account number in a P.O. If he enters an account that does not have an entry in the list of accounts, the update will fail and he should be notified of that."

And I think that the user should not type Account Number - it should be automatically picked. Or if you allow to type it, you should test it right away before even attempting an insert / update.