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

[Martin Bjorklund <[e-mail]>]

Hi,

"Sterne, Jason (Nokia - CA/Ottawa)" <[e-mail]> wrote:
> 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

Assuming all list entries has a 'name', yes, it is the same.

> Or is the * an allowable wildcard for a key value in a predicate ?

"*" is syntactically legal, but is not a wildcard on all values of the
node; it is a wildcard for all nodes.

So if all list entries has a name, A will evaluate to the same nodeset
as B, since "name = *" is a node-set comparison, and the node "name"
will be present in the node set from "*" (node set comparisons are not
always intuitive; read the spec for all details ;-)

> 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 ?

Yes, but if the list is big, the simple "a-list" may be more
efficient, since "count()" will actually count all instances
(modulo existance of optimizations in the evaluator).


/martin