[erlang-questions] Help! Drowning in qlc documentation

[lloyd@REDACTED]

Hello,
 
I really really want to understand qlc. But that's another story. Here's my immediate problem:
 
I have a simple dets db that stores:
 
-record(note, {date, subject, note}).
 
I want to fetch all notes equal or less than a given date. Leveraging Joe Armstrong's do/1 function (Programming Erlang, 2007, p326), I wrote a query function as so:
 
do(Q) ->
 F = fun() -> qlc:e(Q) end,
 {atomic, Val} = mnesia:transaction(F),
 Val.
 
get_notes(Date) ->
    open_notes_db(),
    do(qlc:q([Record || Record <- dets:table(nit_notes), Record#note.date =< Date])),
    close_notes_db().
 
When I run get_notes/1, I get this:
 
36> notes_store:get_notes(Date).
** exception error: no match of right hand side value {aborted,{node_not_running,'nit_notes@REDACTED'}}
 in function notes_store:do/1 (/home/lloyd/nit_host/nit_notes/site/src/notes_store.erl, line 53)
 in call from notes_store:get_notes/1 (/home/lloyd/nit_host/nit_notes/site/src/notes_store.erl, line 59)
 
This suggests that my dets db is not open, but I did that in get_notes/1:
 
open_notes_db() ->
    File = nit_notes,
    {ok, nit_notes} = dets:open_file(File,
    [{keypos,#note.date}, {type,bag}]).
 
 
Which, in the shell, returns:
 
29> notes_store:open_notes_db().
{ok,nit_notes}
 
If I run:
 
qlc:q([Record#note.note || Record <- dets:table(nit_notes), Record#note.date =< Date ])
 
...in the shell, I get:
 
{qlc_handle,{qlc_lc,#Fun<erl_eval.20.90072148>, {qlc_opt,false,false,-1,any,[],any,524288,allowed}}}
 
Yes, I know that this is a query handle. But what in world does it mean? If I plug this mess it into the shell, I get:
 
1> Q = qlc:q([Record#note.note || Record <- dets:table(nit_notes), Record#note.date =< Date ]).
2> qlc:eval(Q).
** exception error: bad argument
 in function dets:safe_fixtable/2
 called as dets:safe_fixtable(nit_notes,true)
 in call from qlc:call/4 (qlc.erl, line 3759)
 in call from qlc:table_handle/3 (qlc.erl, line 2616)
 in call from qlc:setup_le/3 (qlc.erl, line 2381)
 in call from qlc:setup_quals/6 (qlc.erl, line 2480)
 in call from qlc:setup_quals/4 (qlc.erl, line 2438)
 in call from qlc:setup_le/3 (qlc.erl, line 2377)
 in call from qlc:eval/2 (qlc.erl, line 290)
 
Which, again, suggests a problem with my dets table. But I don't know what.
 
Or, is it possible that my problem is with mnesia:transactions/1 in do/1? Docs do say that qlc should work with dets, but I don't see a dets:transactions/1.
 
How can I diagnose and fix?
 
Many thanks,
 
LRP
 
 
 
 
*********************************************
My books:

THE GOSPEL OF ASHES
http://thegospelofashes.com

Strength is not enough. Do they have the courage 
and the cunning? Can they survive long enough to 
save the lives of millions? 

FREEIN' PANCHO
http://freeinpancho.com

A community of misfits help a troubled boy find his way 

AYA TAKEO
http://ayatakeo.com

Star-crossed love, war and power in an alternative 
universe

Available through Amazon or by request from your 
favorite bookstore


**********************************************
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://erlang.org/pipermail/erlang-questions/attachments/20141018/9abcd7a1/attachment.htm>