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POWhat's the best way to prevent adding a record whose primary key is already present in mnesia?
 

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  1. POWhat's the best way to prevent adding a record whose primary key is already present in mnesia?
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    <p>Suppose I've got a simple record definition:</p> <pre><code>-record(data, {primary_key = '_', more_stuff = '_'}). </code></pre> <p>I want a simple function that adds one of these records to a mnesia database. But I want it to fail if there's already an entry with the same primary key.</p> <p>(In the following examples, assume I've already defined</p> <pre><code>db_get_data(Key)-&gt; Q = qlc:q([Datum || Datum = #data{primary_key = RecordKey} &lt;- mnesia:table(data), RecordKey =:= Key]), qlc:e(Q). </code></pre> <p>)</p> <p>The following works, but strikes me as sort of ugly ...</p> <pre><code>add_data(D) when is_record(D, data)-&gt; {atomic, Result} = mnesia:transaction(fun()-&gt; case db_get_data(D#data.primary_key) of [] -&gt; db_add_data(D); _ -&gt; {error, bzzt_duplicate_primary_key} end end), case Result of {error, _} = Error -&gt; throw(Error); _ -&gt; result end. </code></pre> <p>This works too, but is also ugly:</p> <pre><code>add_data(D) when is_record(D, data)-&gt; {atomic, Result} = mnesia:transaction(fun()-&gt; case db_get_data(D#data.primary_key) of [] -&gt; db_add_data(D); _ -&gt; throw({error, bzzt_duplicate_primary_key}) end end). </code></pre> <p>It differs from the above in that the above throws</p> <pre><code>{error, bzzt_duplicate_primary_key}, </code></pre> <p>whereas this one throws</p> <pre><code>{error, {badmatch, {aborted, {throw,{error, bzzt_duplicate_primary_key}}}}} </code></pre> <p>So: is there some convention for indicating this sort of error? Or is there a built-in way that I can get mnesia to throw this error for me?</p>
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