From: "Ara.T.Howard" <ahoward@...>
Date: 2004-06-30T08:02:53+09:00
Subject: Re: [ANN] arrayfields-3.0.0

On Wed, 30 Jun 2004, Ben Giddings wrote:

> Is there any reason you didn't just use either constants or variables like:

yes.

   constants:

     can't be reused - obviously - so

       tuple_a[NAME]

       tuple_b[NAME]

     won't work of NAME is different for both

     this also doesn't work when the relation is modified.  in general there is
     no guaruntee on the order of fields returned to this is never a good
     option.


   variables:

     i like to do this

       name, age, ssn = tuple.values_at 'name', 'age', 'ssn'

     so i already used up those var names - not that i couldn't use 'name_idx',
     etc.  but this starts getting messy fast if you are dealing for four or
     five tables with 10-20 fields each.

lastly there is one nice feature of using the arrayfields method:  say you are
generating a report from a relation using something like you are suggesting

   res = pgconn.exec sql
   tuples, fields = res.tuples, res.fields

   name, age, ssn = %w(name age ssn).map{|f| fields.index f}

   YAML::dump(
     tuples.map do |t|
       Hash[
         'name' => tuple[name],
         'age' => tuple[age],
         'ssn' => tuple[ssn],
       ]
     end
   )

and later the relation is modified to now have a new column you'll need to
modify the code.  whereas if you use the fields this code will work even when
the table is modified

   res = pgconn.exec sql
   tuples, fields = res.tuples, res.fields

   YAML::dump(
     tuples.map do |t|
       t.fields = fields
       fields.inject({}){|h,f| h[f] = t[f]; h}
     end
   )

of course you could write the code to have this feature either way - my point
is just that when you extract the indexes into named vars you limit the way
your code will work as the database add/drops fields...  i was originally also
doing alot of this sort of thing:


   def where_clause tuple, op = ' or '
     clauses =
       tuple.fields.map do |field|
         "#{ field } = '#{ tuple[field] }'"
       end

     clauses.join op
   end

which just started to get painful when the tuple didn't know it's own field
names...  you can imagine doing alot of this stuff if you have to map fields
to var names and pass this mapping into each sql generation method...


cheers.

-a
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