Light loops

Nareeta Martin

Consider find_each for looping through Active Record associations

The standard method for enumerating through groups of objects, both through arrays in Ruby and through Active Record models in Rails, is each.

However, if you are looping over a large amount of data, perhaps all the records for a model in order to backfill data, you can encounter severe memory and speed issues, both with loading and processing large volumes of records.

You should consider the functionality provided by Active Record’s Batches. We’ll demonstrate using find_each.

Instead of…

…looping through lots of Active Record objects using .each:

post.comments.each do |comment|
  # Do stuff with each comment: enqueue a job


.find_each to more efficiently load records from the database:

post.comments.find_each do |comment|
  # Do stuff with each comment: enqueue a job


Using each makes one SQL call to the database and tries to load the entire set of objects into memory and then loop over them. It’s the same as if you’d called post.comments.all.each instead in the first example.

This is a problem in two dimensions. First, the database query may take a long time to execute or may time out. Second, when (or if) it does return data, there’s likely to be significant memory usage as it loads all the records into memory in order to loop over them.

Using .find_each makes a series of more efficient SQL queries (with a bunch of sensible defaults) to retrieve records from the database, which is often a lot more efficient than loading all records into memory at once.

Why not?

If you need to see the records in a particular order, .find_each doesn’t support that, it only uses the primary key to sort during the loop. Also for this reason, find_each doesn’t work reliably if your model has a UUID primary key, as UUIDs aren’t sequential. This means that it’s possible to skip records if new data is being added while you’re looping. (Thanks Iain)

If you need to modify the records in place, this sort of looping isn’t ideal. For example, if you’re running an #update on each record, you’ll be executing a lot of queries. Instead consider more appropriate methods for bulk updating methods, such as #update_all.

Don’t use either the each or find_each version to generate view code. Having a view large collection to loop through in view code, given you don’t know how many records you’ll have, is a recipe for slow pages and poor user experience. Consider using pagination instead.

The Rails guides suggest find_each is only needed for processing a large number of records that wouldn’t fit in memory all at once. If you only need to loop over fewer than one thousand records the regular methods are fine/recommended.

At large scale you’ll need to use even more advanced techniques and you’ll have to move beyond long running loops. Perhaps you want to reduce a high volume of read/write operations on your database or you want to cut down execution time from minutes to hours!

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Last updated on February 27th, 2023