Re: setting default order of entity fetches
Re: setting default order of entity fetches
- Subject: Re: setting default order of entity fetches
- From: Lachlan Deck <email@hidden>
- Date: Sun, 26 Aug 2007 12:28:36 +1000
On 25/08/2007, at 3:22 AM, Chuck Hill wrote:
On Aug 23, 2007, at 5:41 AM, Lachlan Deck wrote:
On 23/08/2007, at 1:38 PM, Chuck Hill wrote:
On Aug 22, 2007, at 6:22 PM, Lachlan Deck wrote:
On 23/08/2007, at 10:46 AM, Chuck Hill wrote:
On Aug 22, 2007, at 5:09 PM, Lachlan Deck wrote:
EOEntity has a restrictingQualifier that's applied to every
fetch for the relevant entity.
Is there any similar mechanism for installing a default
ordering for fetched objects of each entity (e.g., when
following a toMany relationship)? Or any delegate methods
somewhere?
No. And don't override the EOF methods to do this or you risk
messing with EOF. And we all know how that ends up. :-) The
reason for the lack of sorting, as I understand it, is that
EOF would then need to keep the list sorted each time something
was added to it.
It's already keeping them sorted (think about it...). Just not
how I'd like.
No, they are not sorted.
Furthermore, it's impossible to fetch records from a database
without them sorted in some kind of order.
Where on earth did you get that idea from? An unordered list is
not sorted. If there is no ORDER BY clause on a SELECT, the
order in the result set is undefined. It most databases it will
change over time as the query optimizer changes strategies.
I understand the theory - but I'm talking about reality ;-)
Well... I would agree that one of us is. :-)
The point is that once the array has been faulted into memory they
have a fixed order whenever that particular relationship is
accessed. I would simply like to control the order that these
objects are kept in memory so that for the majority of cases no
further ordering is necessary.
Fixed order? Consider:
public NSArray items() // the to-many relationship
EOEnterpriseObject eo = items().objectAtIndex(0);
removeFromItems(eo);
addToItems(eo);
OK, now it is the same relationship, the same members, but a
different order. Your reality based applications ;-) might be
different, but my theoretical ones manipulate to-many relationships.
Ahh, well you're talking about a different kind of app - one where
modifications like the above might occur in the application. So might
I suggest that my reality is not a figment of my imagination and that
not every application is identical in needs ;-)
The one I'm talking about is mostly read-only whereby the majority of
the items displayed on the site are not modified by that app at all
and thus, as I said, their ordering is *not* going to change 95% of
the time until there's a refaulting of the objects after the
timestamp lag has lapsed or the shared ec is automatically reloaded
because of a notification, for example. I would simply like to define
somewhere that the enumeration of objects found at a particular
keyPath would match the ordering of objects that's otherwise been
defined when faulting objects into the shared ec for example.
In order to make your idea work, EOF will have to resort the array
every time addToItems() is called or (and this is a a big one) when
any of the objects in the array have their values changed. That is
going to suck up a lot of processor time. If it does not do that,
the sorting is useless as you can't depend on it.
Like I said above, this is not going to suck up wasted processor time
at all in above app but save processor time being done once and once
only per refaulting. What I was suggesting is that this seems to me
to be a relevant optional 'switch' in the model for some situations.
To many relationships probably should have been implemented as
NSSet not NSArray. Properly they are sets and using NSArray deludes
people into thinking it really is an ordered list. I'd guess that
they used NSArray to avoid the NSSet overhead of verifying that an
object is not in the set before adding it.
Well that would override the delegate methods that seem to allow for
the opportunity to return objects from, e.g., a local cache.
I'm simply wanting to override the arbitrary sort order that the
database wants to return to suit this particular application.
And what if you wanted alternate sorts?
Simply apply the default sorting in the absence of a specific
sort orderings array. Not hard conceptually.
Not sure how that differs from what I suggested and it means two
sorts so additional unneeded load on the machine.
Huh. Where's the second sort? i.e., if you happen to have built a
custom EOFetchSpecification somewhere in your application that
happens to include a non-empty sort orderings array then that
would run as normal. Otherwise, the default one applies whilst
faulting from the database (not in memory).
Fetch? We are talking about to-many relationships. If I have the
objects in a relationship and I want them sorted, why would I fetch
them to sort them? I'd just sort them in memory. But they are
already sorted another way which was a waste of resources.
I've already mentioned the application. For this application you're
talking about a theoretical 1% of the application. Where's the waste?
The standard approach is to have a cover method that sorts the
contents of the relationship and returns it.
Which kinda makes *every* toMany relationship generated from
each EOEntity useless don't you think?
I dunno, I have always found them to be be pretty useful. YMMV.
Like Ken said, ordering is a UI thing.
Personally, I can't see the benefit of continually sorting a
keyPath in memory via some ui option every time you access that
array rather than once from the fetch where possible...
But then how do you know it is still sorted?
Perhaps I've not been clear... this particular application utilises a
standard ordering (defined per entity not per relationship). If there
was a mechanism for simply defining that in the model I'd do so...
think D2W like.
with regards,
--
Lachlan Deck
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