Ruby where with find_each

Question

I am looking at the official Rails documentation which shows how to use the "find_each" method. Here is an example they gave

Person.where("age > 21").find_each do |person|
  person.party_all_night!
end

This processes 1000 records at a time. However, I am still confused. How does this translate to SQL? What happens behind the scenes that allows Ruby to only process 1000 records at a time?

The reason I am sort of confused is because it seems Person.where("age > 21") would execute first, which would return ALL results.

For instance:

Person.where("age > 21").limit(10)

would return all persons in memory first, then give you the first 10, right?

Answer 1

Person.where("age > 21") returns an ActiveRecord relation only. It doesn't return all the results.

Person.where("age > 21").limit(10) does NOT load all the models in memory, that would be awful and unusable. It just loads 10.

find_each doesn't really process 1000 records at a times. It loads 1000 records, and then process each one of them.

Answer 2

I'd suggest running this from the console and looking at the SQL or reading the source code.

For example:

User.find_each(:batch_size => 40) do |user| end
  User Load (1.0ms)  SELECT "users".* FROM "users" WHERE ("users"."id" >= 0) ORDER BY "users"."id" ASC LIMIT 40
  User Load (0.8ms)  SELECT "users".* FROM "users" WHERE ("users"."id" > 96) ORDER BY "users"."id" ASC LIMIT 40
  User Load (0.8ms)  SELECT "users".* FROM "users" WHERE ("users"."id" > 156) ORDER BY "users"."id" ASC LIMIT 40
  User Load (0.8ms)  SELECT "users".* FROM "users" WHERE ("users"."id" > 219) ORDER BY "users"."id" ASC LIMIT 40
  User Load (0.8ms)  SELECT "users".* FROM "users" WHERE ("users"."id" > 272) ORDER BY "users"."id" ASC LIMIT 40
  User Load (0.8ms)  SELECT "users".* FROM "users" WHERE ("users"."id" > 314) ORDER BY "users"."id" ASC LIMIT 40
  User Load (0.8ms)  SELECT "users".* FROM "users" WHERE ("users"."id" > 355) ORDER BY "users"."id" ASC LIMIT 40

Or

bundle show activerecord
point your favorite code editor at that location and find the source