In Rails, what's the difference between find_each and where?

Question

In Rails, both find_each and where are used for retrieving data from Database supported by ActiveRecord.

You can pass your query condition to where, like:

c = Category.where(:name => 'Ruby', :position => 1)

And you can pass batch size to find_each, like:

Hedgehog.find_each(batch_size: 50).map{ |p| p.to_json }

But what's the difference between the following 2 code?

# code 1
Person.where("age > 21").find_each(batch_size: 50) do |person|
  # processing
end

# code 2
Person.where("age > 21").each do |person|
  # processing
end

Does code 1 batch retrieve 50 tuples each time, and code 2 retrieve all tuples in one time? More details explaination is welcomed.

My opinion is:

1. both where and find_each can be used for batch retrieving, but user can define batch size when using find_each.
2. find_each does not support passing query condition.

Please correct me if my understanding is wrong.

Answer 1

An active record relation does not automatically load all records into memory.

When you call #each, all records will be loaded into memory. When you call #find_each, records will be loaded into memory in batches of the given batch size.

So when your query returns a number of records that would be too much memory for the server's available resources, then using #find_each would be a great choice.

It's basically like using ruby's lazy enumeration #to_enum#lazy with #each_slice and then #each (very convenient).