mnesia:transaction aborting

[Peter J Etheridge]

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

[Dániel Szoboszlay]

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 <[hidden email]> 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