Add Load Records In Batches With find_each as a rails til

rails/load-records-in-batches-with-find-each.md

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+# Load Records In Batches With find_each
+
+The base enumerable method offered by Ruby is `#each`. If you need to interact
+with an array of elements, that's a method you'll reach for at some point.
+
+When working with an `ActiveRecord` collection in Rails, you should use the
+[`#find_each`](https://api.rubyonrails.org/v6.1.0/classes/ActiveRecord/Batches.html#method-i-find_each)
+method instead of `#each`. That's because under the hood it batches the records
+that it will load in 1000 at a time. This is important to keep your server's
+resource usage from exploding when requesting a ton of records.
+
+Consider a `users` table that contains 10,000 records that are _active_.
+
+```ruby
+User.where(active: true).each do |user|
+  # do something
+end
+```
+
+With `#each`, all 10,000 records will be loaded into memory at once as
+`ActiveRecord` objects. That's potentially a lot of load on the server's
+available memory. Then imagine the table contains 100,000 or 1,000,000 records.
+This can become a big problem.
+
+```ruby
+User.where(active: true).find_each do |user|
+  # do something
+end
+```
+
+With `#find_each`, which uses
+[`#find_in_batches`](https://api.rubyonrails.org/v6.1.0/classes/ActiveRecord/Batches.html#method-i-find_in_batches)
+under the hood, only 1000 `ActiveRecord` objects get loaded into memory at a
+time.
+
+If you want to exercise more control over the batching, you can use
+`#find_in_batches` directly.