On 11-Aug-09, at 1:45 PM, David Avendasora wrote:
On Aug 11, 2009, at 11:32 AM, Michael Halliday wrote:
Hi List,
Let's say you have in your model:
Customer -->> Transactions
Since a customer could have 1000's or 10000's transactions ...
obviously you wouldn't want to have transactions as a class
property on the Customer EO for performance reasons. Normally you
would never want to get at ALL the transactions for a customer at
once, but you might want to get "Open" transactions or "Flagged"
transactions. So, for example you could have a method on the
Customer EO that just fetches the Transaction with a qualifier as
below:
public NSArray<Transaction> openTransactions() {
return Transaction.fetchTransactions(editingContext(),
Transaction.CUSTOMER.eq(this).and(Transaction.STATUS.eq("Open")),
null);
}
I generally will have a private variable in the EO:
private NSArray<Transaction> openTransactions;
Then the above method would change to:
public NSArray<Transaction> openTransactions() {
if(openTransactions == null) {
openTransactions = Transaction.fetchTransactions(editingContext(),
Transaction.CUSTOMER.eq(this).and(Transaction.STATUS.eq("Open")),
null);
}
return openTransactions;
}
This is pretty much exactly what I do as well. Just didn't know if
there was a "better way" of doing this and managing the cache.
Then you'll need to manually manage at what point you need to
refresh the data stored in the openTransactions variable. I usually
would add openTransactions = null to the methods that add-to or
remove-from the relationship. This is a bit heavy handed though. You
may want to do something more fine-grained if you start running into
performance issues.
Yeah, it can be a bit heavy handed at times ... but I guess it does
the job.
Dave
Now, this is all well and good ... but what's the "best practice"
for caching this result? You wouldn't want to bind this method up
to a WORepetition for example! I'd just be curious to see how
other people are handling these situations. Do you cache this
stuff in the EO itself ... or do you just limit calls to such
methods and cache the return value? I'm basically describing the
role of Core Data's fetched properties.
Cheers,
Michael
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