Re: [yang-doctors] Yang syntax for a globally-unique key?

> On Aug 26, 2020, at 3:21 AM, Ladislav Lhotka <ladislav.lhotka@nic.cz> wrote:
> 
> Kent Watsen <kent@watsen.net> writes:
> 
>> Hi Lada,
>> 
>>>>>>     Assume data model:
>>>>>> 
>>>>>>       +—rw tenants
>>>>>>           +—rw tenant [key]
>>>>>>               +—rw  key        string
>>>>>>               +—rw admins
>>>>>>                   +—rw admin [name]
>>>>>>                        +—rw name     string
>>>> <snip>
>>>> I don't see any augmentation involved in either case. Anyway, I do agree
>>>> that the unique statement should work. Otherwise, section 12.16 of RFC
>>>> 6110 essentially gives what Kent asks for, which is in this case:
>>>> 
>>>>   must "not(preceding-sibling::tenant[current()/admins/admin/name])";
>>> 
>>> Sorry, it should be
>>> 
>>>   must "not(preceding-sibling::tenant[admins/admin/name  =
>>> current()/admins/admin/name])
>> 
>> I thought "preceding-sibling” was for selecting a sibling?  In my example, “tenant” doesn’t have a sibling, so what is selected?  You *are* putting the must  statement on the “tenant” node, right?
> 
> Yes. In your example, "tenant" is a list, and its instances/entries are sibling nodes in the XPath data model. So the rule states that there must not be two "tenant" entries containing admins/admin/leaf with the same value. If this condition is violated, it is reported for the second "tenant" entry.
> 
> The same rule can be written in other ways, too, but the advantage of preceding-sibling:: is that the check is performed only "above the diagonal" of all pairwise combinations of "tenant" entries.

I see, that makes sense now.  I didn’t realize that "preceding-sibling” selected all siblings.

Pyang validated the corrected expression above, but yanglint threw an error, as described by: https://github.com/CESNET/libyang/issues/1180 <https://github.com/CESNET/libyang/issues/1180>.

I have not yet tested with Yangson.  Is it known if Yangson supports "preceding-sibling”?  Looking at https://yangson.labs.nic.cz/xpath.html <https://yangson.labs.nic.cz/xpath.html> I see this:

    NotSupported – if the input XPath expression contains a feature that
    isn’t supported by the implementation, such as the preceding:: axis.

Kent

> Lada
> 
> -- 
> Ladislav Lhotka 
> Head, CZ.NIC Labs
> PGP Key ID: 0xB8F92B08A9F76C67
> 
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