Re: check whether a relationship is empty, efficiently
Re: check whether a relationship is empty, efficiently
- Subject: Re: check whether a relationship is empty, efficiently
- From: Ramsey Gurley <email@hidden>
- Date: Thu, 04 Feb 2016 10:29:25 -0700
I’ve done this before with a derived attribute to count to many relationship objects.
https://github.com/nullterminated/ponder/blob/master/ERR2d2w/Sources/er/r2d2w/ERR2d2w.java#L111
I was very green when I wrote that, so it’s sorta crappy. It doesn’t support compound FKs. Despite the comment, count(*) is not really MySQL specific. But it worked for me then. That allowed me to display (and sort) to many relationships as a count of objects in that relationship.
Derived counts only update on fetch, so they might be too stale in your case. You could also try a ERXExistsQualifier. That is sorta buggy, but it has worked for me in limited usage. It probably won’t be much help unless you use it in a fetch though. In memory, I suspect it just triggers a fault like anything else.
On Feb 3, 2016, at 6:34 PM, OC <email@hidden> wrote:
> Hello there,
>
> I'd bet this must be a WebObjects 101, but I can't see the best answer: is there an efficient way of checking whether an eo.someRelationship is empty?
>
> So far I simply check eo.someRelationship.count==0, but that might fire a fault, which is not exactly efficient if the only need is to know whether the relationship is empty or not.
>
> Of course I could rig myself something like “if already fetched in, simply check array count; if still fault, fetch max one object and see whether there's any...”, but this looks like a ubiquitous task enough it simply _must_ be done already (and in a far better way than I would do it myself)?
>
> Thanks for any advice,
> OC
>
>
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