[erlang-questions] Strategies for updating Mnesia tables

[Igor Ribeiro Sucupira]

On Fri, May 9, 2008 at 12:07 PM, Alexander Lamb
<alexander.lamb@REDACTED> wrote:
> Hello,
>
>  Although Mnesia columns can store any kind of Erlang terms, it can
>  happen that we need to add a column after an application is deployed.
>  I understood (and tried) how to update the table.
>
>  However, on the contrary to traditionnal SQL databases, this will have
>  an impact on my code. Indeed, something like:
>
>  ps_get_profiles(System_Name,Production_Type) ->
>         F = fun() -> mnesia:match_object(profiles,{profiles,
>  {'_','_','_'},System_Name,Production_Type,'_','_','_'},read) end,
>         case mnesia:transaction(F) of
>                 {atomic, Records}       -> {ok, Records};
>                 {aborted, Reason}       -> {error, Reason}
>         end.
>
>  Will simply find 0 records once I add a column, hence failing silently.

Well... the approach below is VERY ugly, but it should make you able
to add columns without recompiling.

ps_get_profiles(System_Name,Production_Type) ->
    F = fun() -> qlc:eval(qlc:q([ProfilesRecord ||
                                          ProfilesRecord <-
mnesia:table(profiles),
                                          System_Name =:= element(3,
ProfilesRecord),
                                          Production_Type =:=
element(4, ProfilesRecord),
                                          is_tuple(element(2, ProfilesRecord)),
                                          size(element(2,
ProfilesRecord)) =:= 3]))
          end,
    case mnesia:transaction(F) of
        {atomic, Records}       -> {ok, Records};
        {aborted, Reason}       -> {error, Reason}
    end.


But, as I'm pretty new to Erlang (couple of weeks), I suppose there
must be a more beautiful way of doing that.

Igor.

>  Is there a way to match against a variable number of columns, giving
>  the first columns patterns. Something like:
>
>         F = fun() -> mnesia:match_object(profiles,{profiles,
>  {'_','_','_'},System_Name,Production_Type,'_','_','_', * },read) end,
>
>  See the "*" at the end of the match_object match part.
>
>  What is the strategy when updating software that necessitates adding
>  or changing Mnesia tables.
>
>  Thanks,
>
>  Alex
>  --
>  Alexander Lamb
>  Founding Associate
>  RODANOTECH Sàrl
>
>  4 ch. de la Tour de Champel
>  1206 Geneva
>  Switzerland
>
>  Tel:  022 347 77 37
>  Fax: 022 347 77 38
>
>  http://www.rodanotech.ch
>
>
>
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