I have a NSFetchedResultsController that gets me a collection of 'transaction'-esque objects that match a certain criteria — a sale over $100, for example. Each transaction has a unique integer identifier that corresponds to the relative time of the transaction (transaction 23 came before transaction 24). I have no problem displaying the results in a UITableView Controller. What I'd like to do is also show the transaction that occurred right before each of these sales over $100, i.e.:
Transaction - $93 (id = 1)
Transaction - $101 (id = 2)
Transaction - $12 (id = 13)
Transaction - $243 (id = 14) etc.
and then display this list of results in a TableView. There is no chance that two transactions over $100 happened after one another so there is no worry about duplicated entries. Is there any way to do this? I was searching around on how to do this in SQL for inspiration, but there were only answers on how to get 'rows' immediately before a given individual result row, not a set of them.
Any ideas?
You can do it with two fetches and one sorting operation (short of fetching all transactions and filtering in memory).
First fetch all transactions above 100$. Then extract the ids and fetch the corresponding preceding transactions. Merge and sort.
// first fetch with this predicate
NSPredicate(format:"amount > 100")
// get all previous ids
let previousIDs = result.map { $0.id - 1 }
// fetch the other records with this predicate
NSPredicate(format:"id in %@", previousIDs)
// merge & sort
let finalResult = (result + result2).sort { $0.id < $1.id }