Re: pvput with Sub-structures

[Michael Davidsaver via Tech-talk <tech-talk at aps.anl.gov>]

Hello Dennis,

On 1/5/26 4:41 AM, Dennis Neidherr via Tech-talk wrote:

Dear all,

Happy new year!
At the moment I am playing with pvAccess (and p4p) to see how this is working.
So for my tests, I have successfully built the pvxs-support-module and created an application which uses it.

In the db File I added two new records:
record(ai,"myTest1")
{
    field(SCAN, "Passive")
    field(VAL, 2)
}
record(ai,"myTest2")
{
    field(SCAN, "Passive")
    field(VAL, 4)
}

And I added a json-File, which is loaded at the startup of the IOC:
{
    "grp:name": {
        "X": {"+channel":"myTest1.VAL"},
        "Y": {"+channel":"myTest2.VAL"}
    }
}

...
So I tried it:
pvput grp:name X='{"value":"77"}'
This is also not working (The error is just "No fields changed"). 

In this case, the admittedly unclear message is telling you that no fields can be changed because not are changeable.  The group definition above does not define any "putorder".  Try adjusting your group definition to:

{
   "grp:name": {
       "X": {"+channel":"myTest1.VAL", "+putorder":0},
       "Y": {"+channel":"myTest2.VAL", "+putorder":1}
   }
}

This specifies the order in which members of the group are written and processed.

The PVXS documentation has an illustrated example of a writable group definition.

https ://epics-base.github.io/pvxs/qgroup.html#understanding-groups

From my internet search I got this link:
https://urldefense.us/v3/__https://mrkraimer.github.io/website/developerGuide/pvTools/pvTools.html__;!!G_uCfscf7eWS!buHkU2vY2nC-8z2UrnhpcT58Z50NjPTQVfA3ka4Pe3TQwIPKPUKS3b-mB8S1-9ocmlLnmM0j4F50C1K5Vi01tz0d$
So I tried also:
pvput -r "X" grp:name X='{"value":"77"}'

But again the same "error" is shown.

I also tried it in python (p4p library):
...
        v = await ctx.get("grp:name")
        print("Old value: " + str(v.X.value))
        v.X.value = 42
        await ctx.put("grp:name", v)
        u = await ctx.get("grp:name")
        print("New value: " + str(u.X.value))
...
But again: The value is not set and the error is "No fields changed".
Since this error is a bit unspecific, I have no ideas how to proceed.

Thanks in advance for any help,
Greetings Dennis