Mnesia and uniqueness on two keys

ke.han <>
Wed Jul 13 13:20:48 CEST 2005


Joel Reymont wrote:
> I have a record defined like this:
> 
> -record(tick_data, {
>       symbol,
>       date,
>       ticks % tuple
>      }).
> 
> I create the table like this:
> 
> mnesia:create_table(tick_data,
>                  [
>                   {disc_copies, Nodes},
>                   {index, [date]},
>                   {type, set},
>                   {attributes, record_info(fields, tick_data)}
>                  ])
> 
> My intent is to have a table where multiple records with the same  
> symbol but different dates can coexist. Will the above work or do I  
> need to create a bag table?
> 
> In other words, does Mnesia expect uniqueness on each keys or do both  
> keys work as a composite?
> 
>     Thanks, Joel

Joel,
I'm a novice, but my understanding is that the first field to your 
record is always the key.  So if you want to key by symbol and date, you 
  have the first field as a tuple {symbol, date} and the second field is 
ticks.  This means you get the keying and querying as you desire.  You 
simply have to unpack the tuple in order to get at the symbol or date 
values.
I would appreciate if someone more experienced can confirm/deny my 
understanding.
thanks, ke han


> 
> -- 
> http://wagerlabs.com/uptick
> 
> 
> 
> 




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