Get "un"related records function ?

I have a multiselect in Products to relate products to accounts. I use a custom function to relate them automatically based on a field.

What I need to know is which products are unrelated to accounts. I can use getRelatedRecords to get the ones that are related, but I need is the opposite: get products that are unrelated.

I guess there is no a getUnrelatedRecords function. How can I accomplish that ?

Thank you.