RANT AGAINST DOCUMENTATION:
I have searched (a lot) and found various other pleas for help of the form, "I
am trying to do a Core Date fetch on an SQL-backed store and it is failing
because something is wrong with my predicate." I thought some investigate
could promote my plea beyond this basic level, but I seem to be hitting the
same wall-of-no-information as everyone else. This wall seems to built around
Apple believing it's official Core Data docs adequately describes the issues
with fetch predicates on SQL by stating, "You cannot necessarily translate
“arbitrary” SQL queries into predicates". Translation: "The issue with
predicates and SQL is that there are issues." Thanks for all the help, docs.
BEGIN PROBLEM AND QUESTION:
So, here is my plea for help. I have a predicate of the form: "<character
string> BEGINSWITH path". In other words, I am searching entities that possess
a (string) property "path" to look for one whose path is a prefix of some
string (which is inserted into the predicate via the fetch-request-template
variable substitution). This fetch executes just fine on an XML store. The
same fetch on an SQL store yields: unimplemented SQL generation for predicate
("/Volumes/MacHD/Applications/Utilities" BEGINSWITH path).
My guess (given the lack of explicit documentation) is that SQL itself (or,
perhaps, just Core Data's use of SQL) does not allow the left-hand-side to be a
literal string, rather, it wants the left-hand-side to be a property key or
key-path. Anyone know if this is actually the case?
So far, in the world of "path has to be on the left-hand-side" I have not
formulated a predicate that can determine if path is a prefix of the supplied
string. Yes, I can fetch all entities from the DB and then do an in-memory
search amongst the paths. Just hoping I could have the fetch do all the work.
Mark Sanvitale
Real Networks
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