OK I think I understand why it doesn't work. I'm attaching a simple
patch that hopefully solves the issue. I've put comments in the commit
message. Please let me know if anything is not clear or if you see
issues.

Eric

On Thu, Jun 7, 2018 at 11:18 PM Eric Danan <eric.danan@u-cergy.fr> wrote:
>
> Thanks.
>
>> As far as I know it is not currently possible to get property
>> auto-completion from the property list of the file itself.
>
>
> I don't understand: when I call "org-set-property" directly from the file (C-c C-x p), it does get all the property values in the file and proposes them for completion. I am probably missing your pint, could I ask you to explain?
>
> Only when I use the "%^{prop}p" capture template element are the values not offered. I don't understand why because looking at the code of "org-capture-fill-template", it seems to simply call "org-set-property" for this element.
>
>> but you could still make use of the stock property in the template and auto-complete
>> from there
>>
>> For instance:
>>
>> ("T" "TEST" entry (file+headline"/tmp/test.org" "drill")
>>         "** TEST  \n :PROPERTIES:\n :Effort: %^{prompt|0:10|0:20|0:30|1:00|2:00|3:00}\n :END: \n %^{PROMPT} " )
>>
>> see the rest of the thread for more details http://lists.gnu.org/archive/html/emacs-orgmode/2011-10/msg01339.html
>
>
> Yes I've read the rest of the thread, but that's not always feasible. In my use case, the property values are titles of academic journals, I can't list them all inside a "%^{prompt}" template element, I need them to be fetched from the file.
>
> Thanks again.
>
> Eric