Forum: Ruby on Rails Rails finder method efficiency question

Consider the following:

anything = WorkoutExercise.find_all_by_exercise_id(params[:id])
if (anything.empty?)
  do_something
else
  do_something_else
end



Is the array returned in 'anything' an intelligent proxy to the
database?
If the result set was a terabyte of data am I in trouble?  If so, what
should I do?

Thanks,

Cris

On Friday, February 8, 2013 4:28:58 AM UTC+8, Ruby-Forum.com User wrote:
>
>
> Is the array returned in 'anything' an intelligent proxy to the
> database?
>

Any time you call .all, .first or find (including find_all_by_...) 
you're
actually  executing the query and unless you use find_each the entirety 
of
the results will be materialised. The result is an actual array (or an
active record object in the case of .first), not a proxy to anything.


> If the result set was a terabyte of data am I in trouble?  If so, what
> should I do?
>
> I'd do something like

if  WorkoutExercise.where(:exercise_id =>(params[:id]).exists?
...
end

assuming you don't actually need the result set. If you do need to do 
stuff
with the result set, use find_each to process the query in batches.

Fred

Thanks,

Fred,

Thank you.

We tested what you gave =us with ActiveRecord logging to STDOUT and with
this code:

   puts 'hi'
    e = WorkoutExercise.where(:exercise_id =>(params[:id]))
    puts "after"
    e.exists?
    puts "last one"


We saw this:

hi
after
  Primary Key (15.0ms)  SELECT cc.column_name FROM all_constraints c,
all_cons_columns cc WHERE c.owner = 'BROWNBAG' AND c.table_name =
'WORKOUT_EXERCISES' AND c.constraint_type = 'P' AND cc.owner = c.owner
AND cc.constraint_name = c.constraint_name
  workout_exercises Columns (16.0ms)  SELECT column_name AS name,
data_type AS sql_type, data_default, nullable, virtual_column,
hidden_column, DECODE(data_type, 'NUMBER', data_precision, 'FLOAT',
data_precision, 'VARCHAR2', DECODE(char_used, 'C', char_length,
data_length), 'RAW', DECODE(char_used, 'C', char_length, data_length),
'CHAR', DECODE(char_used, 'C', char_length, data_length), NULL) AS
limit, DECODE(data_type, 'NUMBER', data_scale, NULL) AS scale FROM
all_tab_cols WHERE owner = 'BROWNBAG' AND table_name =
'WORKOUT_EXERCISES' AND hidden_column = 'NO' ORDER BY column_id
  WorkoutExercise Exists (157.0ms)  SELECT 1 AS one FROM
"WORKOUT_EXERCISES" WHERE "WORKOUT_EXERCISES"."EXERCISE_ID" = 980190962
AND ROWNUM <= 1
last one


So sure enough the query waits until exists? is called!

Thanks!

Cris