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

Answer 2

You can use SQL EXISTS keyword in elegant Rails-ish manner using Where Exists gem:

members.where_exists(:responses).count

Of course you can use raw SQL as well:

members.where("EXISTS" \
  "(SELECT 1 FROM responses WHERE responses.member_id = members.id)").
  count

Answer 3

You can use includes and then test if the related response(s) exists like this:

def surveys_completed
  members.includes(:responses).where('responses.id IS NOT NULL')
end

Here is an alternative, with joins:

def surveys_completed
  members.joins(:responses)
end

The solution using Rails 4:

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

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Alternative solution using activerecord_where_assoc: This gem does exactly what is asked here: use EXISTS to to do a condition. It works with Rails 4.1 to the most recent.

members.where_assoc_exists(:responses)

It can also do much more!

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Answer 4

You can also use a subquery:

members.where(id: Response.select(:member_id))

In comparison to something with includes it will not load the associated models (which is a performance benefit if you do not need them).