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

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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