Rails finder method efficiency question

Rails
1

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

2

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,

3

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