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<  Erlang  ~  New to Erlang: problem with mnesia update query
LRP
Posted: Wed Oct 08, 2008 8:15 pm Reply with quote
Joined: 08 Oct 2008 Posts: 8 Location: Boston, MA.
Hello,

Following the pattern in Getting Started with Mnesia:

http://www.erlang-fr.org/erlang.org/lib/mnesia-4.1/doc/html/Mnesia_chap2.html

I've written the following update query in my news server

updateHead(NewsID, Head) ->
%% update news item headline
mnesia:transaction(
fun() ->
[R] = mnesia:read({news, NewsID, write}),
New = R#news{head = Head},
mnesia:write(New)
end ).

Compiles fine. But doesn't update.

No doubt I'm doing something dumb. Can some kind soul please steer me right?

Here are the related elements in my server source:

handle_call({update_head, NewsID, Head}, _From, State) ->
updateHead(NewsID, Head),
{reply, ok, State};

updateNewsHead(NewsID, Head) ->
gen_server:call(?SERVER, {update_head, NewsID, Head}).


-export([start_link/0, insertNewsItem/2, updateNewsHead/2, listNews/0, selectNewsItem/1, selectNewsByDate/2, shutdown/0]).

Table def:

init_newsStore() ->
% Initialize database
mnesia:create_schema([node()]),
mnesia:start(),
try
mnesia:table_info(news, type)
catch
exit: _ ->
mnesia:create_table(news, [
{attributes, record_info(fields, news)},
{type, set},
{disc_copies, [node()] }
])
end.


Comment: Gee I wish there was one mnesia tutorial with a half a dozen or so SIMPLE examples each of insert/select/update/delete queries. I've been scrambling all over the net to figure out how to write this one function.

Many thanks in advance,

LRP
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Roux Viljoen
Posted: Tue Nov 04, 2008 11:46 am Reply with quote
Joined: 04 Nov 2008 Posts: 3 Location: London
LRP,

I believe you used the mnesia:read/3 incorrectly.

Code:

[R] = mnesia:read({news, NewsID, write}),


instead of...

Code:

[R] = mnesia:read(news, NewsID, write),


Once its called correctly you shouldn't have any more problems except if you have no entries that match with the NewsID.
To prevent yourself from getting an error, you should use a "Case" to cater for a [] return.

Hope that helps Smile
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