How to solve error Sequence contain more than one element?

Single() or FirstOrDefault()

I have this code:

select new PartsInvoiceItemData
                                                {
                                                    Id = pii.Id,
                                                        Model =
                                                    (
                                                       <Query here>
                                                       ).Single(),

Error: Sequence contain more than one element.

Solution: Change Single() to FirstOrDefaut() because the query returns more than one results