On 24 May 2008, at 21:47, Ben Trumbull wrote:
At 2:56 AM -0700 5/24/08, [EMAIL PROTECTED] wrote:
I have a core data object graph using an SQLite store where several
entities have an attribute containing a date (eg, birthDate). This
is stored as a date type.
I am currently implementing an NSPredicateEditor for building smart
groups. Each smart group simply saves this predicate to an
attribute and then fetches using that.
The problem is that I want to offer to filter by ('days since
birth' == 7), as well as (birthDate == 01/01/01). This is where I
have hit a
wall. I can't set a dynamic transient property for daysSinceBirth as
these can't be used in fetch requests, and I can't convert the number
of days to an absolute date as this loses the dynamic-calculation
part
of the number.
This is the part I'm not following. Why don't you transform:
'days since birth' == 7 into 'birthDate == $x' where $x =
'currentDate - 7 days'
Generally speaking, you always want to work with absolute time
(seconds since reference date, UTC) in the database. The only place
you should use calendar, human legible dates, or time zones is with
entities like users' Preferences or user's active Session.
Obviously your UI layer needs to represent dates in a way your
customers will appreciate.
Calendar properties are political and historical issues as well as
geographically relative. Calendars make the Unicode spec look like
riding a big wheel. They are generally not useful to store in the
database.
Nothing stops you from using the NSPredicateEditor with 'fake'
properties and then grabbing the predicate it makes and transforming
it into another one. Pulling apart an NSPredicate tree is tedious,
but not hard.
--
-Ben
I didn't want to transform and store an absolute date, as the
predicate should always show 7 days ago, no matter what the date
today. I have got an example working similarly to how you suggest
though:
It sets a constant value for the left expression and the number of
seconds to adjust by in the right expression. This is saved to the
store exactly as-is. When it is retrieved, that constant key is
detected and the subpredicate replaced with a key path and absolute
date suitable for fetching with.
Thankfully, this app is *highly* region-specific, so calendar
differences are of little account.
Thanks for the advice though. It's not as pretty as I hoped, but it
seems to work quite well.
Ben.
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