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TODAY I LEARNED

Beware of calling #count on Active Record relations!

Given code like this:

records = Record.includes(:related).all # Eager-loads to prevent N+1 queries...
records.each do |record|
  puts record.related.count # => ... but this produces N+1 queries anyway!
end

If you run this, you'll notice you get an N+1 queries problem, even though we're using #includes. This happens because of record.related.count. Remember, records.related here is not an Array but an instance of CollectionProxy and its #count method always reaches out to the database. Use #length or #size instead to solve this issue.

records = Record.includes(:related).all
records.each do |record|
  puts record.related.length # Problem solved!
end

Learned by kevin-perez on Feb 18, 2021

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