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Many thanks, Ulf,</p>
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That did the trick.</p>
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Best wishes,</p>
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LRP</p>
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-----Original Message-----<br>
From: "Ulf Wiger" <ulf@feuerlabs.com><br>
Sent: Sunday, October 19, 2014 3:33am<br>
To: lloyd@writersglen.com<br>
Cc: "erlang questions" <erlang-questions@erlang.org><br>
Subject: Re: [erlang-questions] Help! Drowning in qlc documentation<br>
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<div>The {aborted,{node_not_running,'<a href="mailto:nit_notes@127.0.0.1">nit_notes@127.0.0.1</a>’}} message comes from mnesia, as you’re trying to run a mnesia transaction without actually having a mnesia database.</div>
<div>You don’t need mnesia at all to use QLC. Just execute qlc:e(Q) directly.</div>
<div>BR,</div>
<div>Ulf W</div>
<br>
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<div>On 18 Oct 2014, at 17:46, <a href="mailto:lloyd@writersglen.com">lloyd@writersglen.com</a> wrote:</div>
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Hello,</div>
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I really really want to understand qlc. But that's another story. Here's my immediate problem:</div>
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I have a simple dets db that stores:</div>
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-record(note, {date, subject, note}).</div>
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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:</div>
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do(Q) -><br>
F = fun() -> qlc:e(Q) end,<br>
{atomic, Val} = mnesia:transaction(F),<br>
Val.</div>
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get_notes(Date) -><br>
open_notes_db(),<br>
do(qlc:q([Record || Record <- dets:table(nit_notes), Record#note.date =< Date])),<br>
close_notes_db().</div>
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When I run get_notes/1, I get this:</div>
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36> notes_store:get_notes(Date).<br>
** exception error: no match of right hand side value {aborted,{node_not_running,'<a href="mailto:nit_notes@127.0.0.1">nit_notes@127.0.0.1</a>'}}<br>
in function notes_store:do/1 (/home/lloyd/nit_host/nit_notes/site/src/notes_store.erl, line 53)<br>
in call from notes_store:get_notes/1 (/home/lloyd/nit_host/nit_notes/site/src/notes_store.erl, line 59)</div>
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This suggests that my dets db is not open, but I did that in get_notes/1:</div>
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open_notes_db() -><br>
File = nit_notes,<br>
{ok, nit_notes} = dets:open_file(File,<br>
[{keypos,#note.date}, {type,bag}]).</div>
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Which, in the shell, returns:</div>
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29> notes_store:open_notes_db().<br>
{ok,nit_notes}</div>
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If I run:</div>
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qlc:q([Record#note.note || Record <- dets:table(nit_notes), Record#note.date =< Date ])</div>
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...in the shell, I get:</div>
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{qlc_handle,{qlc_lc,#Fun<erl_eval.20.90072148>, {qlc_opt,false,false,-1,any,[],any,524288,allowed}}}</div>
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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:</div>
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1> Q = qlc:q([Record#note.note || Record <- dets:table(nit_notes), Record#note.date =< Date ]).</div>
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2> qlc:eval(Q).</div>
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** exception error: bad argument<br>
in function dets:safe_fixtable/2<br>
called as dets:safe_fixtable(nit_notes,true)<br>
in call from qlc:call/4 (qlc.erl, line 3759)<br>
in call from qlc:table_handle/3 (qlc.erl, line 2616)<br>
in call from qlc:setup_le/3 (qlc.erl, line 2381)<br>
in call from qlc:setup_quals/6 (qlc.erl, line 2480)<br>
in call from qlc:setup_quals/4 (qlc.erl, line 2438)<br>
in call from qlc:setup_le/3 (qlc.erl, line 2377)<br>
in call from qlc:eval/2 (qlc.erl, line 290)</div>
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Which, again, suggests a problem with my dets table. But I don't know what.</div>
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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.</div>
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How can I diagnose and fix?</div>
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Many thanks,</div>
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LRP</div>
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<div>Ulf Wiger, Co-founder & Developer Advocate, Feuerlabs Inc.</div>
<div><a href="https://urldefense.proofpoint.com/v1/url?u=http://feuerlabs.com&k=ZVNjlDMF0FElm4dQtryO4A%3D%3D%0A&r=F3fV0VibzuPLq4QAFCq8Kw%3D%3D%0A&m=JtLrESuxvAnJLOT4Xj%2FPAkSy%2FHsM42%2F5D5pa92dVxDU%3D%0A&s=7c347076676a5bfeacfc2688d0966fe808cf56e79c2e63b9631467009262a5d1">http://feuerlabs.com</a></div>
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