Rails active record querying association with 'exists'

Question

I am working on an app that allows Members to take a survey (Member has a one to many relationship with Response). Response holds the member_id, question_id, and their answe

Answer 1

If you are on Rails 5 and above you should use left_joins. Otherwise a manual "LEFT OUTER JOINS" will also work. This is more performant than using includes mentioned in https://stackoverflow.com/a/18234998/3788753. includes will attempt to load the related objects into memory, whereas left_joins will build a "LEFT OUTER JOINS" query.

def surveys_completed
  members.left_joins(:responses).where.not(responses: { id: nil })
end

Even if there are no related records (like the query above where you are finding by nil) includes still uses more memory. In my testing I found includes uses ~33x more memory on Rails 5.2.1. On Rails 4.2.x it was ~44x more memory compared to doing the joins manually.

See this gist for the test: https://gist.github.com/johnathanludwig/96fc33fc135ee558e0f09fb23a8cf3f1