Re: [netmod] YANG 'must' Xpaths, predicates and wildcards

[Martin Bjorklund <[e-mail]>]

"Sterne, Jason (Nokia - CA/Ottawa)" <[e-mail]> wrote:
> Thanks Alex.  Sorry about those sloppy mistakes.  I agree about the
> ../a-list and I should have said count > 0.
> 
> In the 2nd part of my email, my intention was to only allow foo to be
> configured if a-list has at least one entry configured.  So I don't
> think min-elements 1 would work.  I don't want to always require an
> entry in a-list.  I only want to require one if foo is configured.
> 
> I guess this also achieves the same thing right ?
>       must "../a-list[entry=*]";

Yes, if all a-list entries has a node called "entry".  But if that't
what you want, do:

       must "../a-list[entry]";


> If foo has a default value, then does that mean the "must" is
> evaluated even if foo is deleted from the config ?
>     leaf foo {
>       must "../a-list";   <- always evaluated because of default ?
>       type uint16;
>       default 5;
>     }
> If the must is always evaluated then it would be the equivalent of
> having min-elements 1 in a-list.


Correct.


/martin

> 
> Rgds,
> Jason
> 
> From: Alex Campbell [mailto:[e-mail]]
> Sent: Tuesday, March 27, 2018 9:57 PM
> To: Sterne, Jason (Nokia - CA/Ottawa) <[e-mail]>;
> netmod@ietf..org
> Subject: Re: YANG 'must' Xpaths, predicates and wildcards
> 
> 
> Hi,
> 
> 
> 
> For one thing, it should be ../a-list since a-list is not a child of
> foo.
> 
> Also - if foo is not configured and has no default value, then any
> must expressions in foo are not evaluated because it is not part of
> the "accessible tree". (I tested this in ConfD)
> 
> Apart from these issues, yes it will behave as you expect - it will
> fail if a-list contains no entries.
> 
> 
> 
> must "count(a-list) > 1"; is not equivalent since it requires at least
> two entries.
> 
> 
> 
> However, you can more simply add a min-elements 1; statement to a-list
> to achieve the same goal - no XPath required.
> 
> 
> 
> ________________________________
> From: netmod <[e-mail]<mailto:[e-mail]>>
> on behalf of Sterne, Jason (Nokia - CA/Ottawa)
> <[e-mail]<mailto:[e-mail]>>
> Sent: Wednesday, 28 March 2018 1:10 p.m.
> To: [e-mail]<mailto:[e-mail]>
> Subject: [netmod] YANG 'must' Xpaths, predicates and wildcards
> 
> Hi all,
> 
> I'm pretty sure that this xpath (e.g. in a must statement) isn't
> correct:
> 
>               (A) ../container-a/list-b[name=*]/some-leaf
> 
> and should just be this instead:
> 
>               (B) ../container-a/list-b/some-leaf
> 
> Or is the * an allowable wildcard for a key value in a predicate ?
> 
> I also had a question about whether the following "must" correctly
> checks that at least one entry exists in a-list.
> 
>   container c1 {
>     leaf foo {
>       must "a-list";
>       type uint16;
>     }
>     list a-list {
>       key "entry";
>       leaf entry {
>         type uint16;
>       }
>       leaf another-entry {
>         type uint32;
>       }
>     }
>   }
> 
> I think I could also replace that must with the following:
>       must "count(a-list) > 1";
> but does must "a-list"; achieve the same thing ?
> 
> Rgds,
> Jason