EdwardDiehl / last_n_per_user_lateral_join

Examples of fetching the "last n per user" with lateral join

Have you ever needed to get the most recent N posts for each user in rails, but didn't know how to do it without using map?

Or maybe something similar like:

• The first or last X comments for each post
• The first or last Y payments for each customer
• The first or last Z reviews for each customer

Sometimes could be ok to just fetch all elements and filter with ruby, but sometimes it is not possible. Also it can cause n+1 queries if your are not careful.

Here I want to show you how you can solve this problem using a lateral join.

Check the example.rb file.

How to run the examples

1. Install the dependencies with bundle install.

2. Run the examples with bundle exec run_playground example.rb

Active Record Playground Runner

This example uses the Active Record Playground Runner by bhserna