Apple

Derived attributes never update until you do a refreshing fetch. So while you can sort of do this, derived attributes will never meet your needs here.

I can’t think of a way to do that that isn’t a lot of code and prone to inconsistencies with the relationship data.

Chuck

From: <webobjects-dev-bounces+chill=email@hidden> on behalf of OC <email@hidden>
Date: Monday, August 22, 2016 at 6:36 AM
To: WebObjects-Dev Mailing List <email@hidden>
Subject: derived attribute to sum a relationship?

Hello there,

is there a way to define a derived attribute to sum a relationship's attribute?

Presumed I have

- entity A with PK column C_ID, relationship:N "ab" leading to entity B

- entity B with FK column C_A_ID (which forms the relationship) and integer column C_X representing attribute "x"

is it possible to define a derived attribute "abSum" of A, which would sum all "x"'s of those B rows, which are -- through their C_A_ID foreign key -- related to a row in A?

Normally, of course, I would implement it Java-side, like this

===

class AEntityClass ... {

int abSum() {

int s=0; for (b in ab()) s+=b.x; return s

}

===

but this time, I would need to do that SQL-side, for (a) speed, (b) memory (so that all B rows do not need to be fetched), (c) so that the attribute can be used in qualifiers when fetching, e.g.,

===

NSArray objs=ec.objectsWithFetchSpecification(new EOFetchSpecification("A",EOQualifier.qualifierWithQualifierFormat("abSum > 100",null),null))

===

I have tried to define it as the most obvious "select sum(bt.C_X) from B_TABLE bt where bt.C_A_ID = C_ID", but I bumped into numerous problems, from ambiguity of C_ID up to the fact that EOF caches the fetched value and does not recache when the relationship objects change, and thus I am getting obsolete values.

Is there a way to do that properly?

Thanks,

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	>derived attribute to sum a relationship? (From: OC <email@hidden>)