Re: mnesia:transaction aborting

If there are 0 records in the pw_usr_record table, the wread will return
[], not [Usr], so your transaction will fail due to case_clause.

Or am I missing something?

Cheers,
Daniel


On Mon, 25 Jan 2021 at 03:57, Peter J Etheridge <[email protected]>
wrote:

> Dear Fellow Erlangers,
>
> I run;
>
> mnesia:transaction(fun() ->
>         case mnesia:wread({pw_usr_record, pw_un}) of
>             [Usr] ->
>                 Usr = #pw_usr_record{pw_un = Username, pw_pas = Password},
>                 mnesia:write(pw_usr_record, Usr, write),
>             catch mnesia:abort("aborted")
>         end
>     end),
>
> result;
>
> mnesia:info().
> ---> Processes holding locks <---
> ---> Processes waiting for locks <---
> ---> Participant transactions <---
> ---> Coordinator transactions <---
> ---> Uncertain transactions <---
> ---> Active tables <---
> pw_usr_record  : with 0        records occupying 305      words of mem
> schema         : with 2        records occupying 533      words of mem
> ===> System info in version "4.17", debug level = verbose <===
> opt_disc. Directory
> "/home/kanga/Documents/ebin/include/priv/src/Mnesia.nonode@nohost" is
> used.
> use fallback at restart = false
> running db nodes   = [nonode@nohost]
> stopped db nodes   = []
> master node tables = []
> remote             = []
> ram_copies         = [pw_usr_record]
> disc_copies        = [schema]
> disc_only_copies   = []
> [{nonode@nohost,disc_copies}] = [schema]
> [{nonode@nohost,ram_copies}] = [pw_usr_record]
> 3 transactions committed, 1 aborted, 0 restarted, 2 logged to disc
> 0 held locks, 0 in queue; 0 local transactions, 0 remote
> 0 transactions waits for other nodes: []
> ok
>
> As shown above, instead of writing (0 records shown) the transaction is
> aborting (1 aborted).
>
> Could the Variable be out of scope?
>
> Where else might I look?
>
> Thank you in advance,
>
> Peter
>
>
>
>
>