Be careful when checking scopes for blankness

Today I stumbled across a pretty harmless-looking query in our application which turned out to be pretty harmful and caused huge memory usage as well as downing our passenger workers by letting requests take up to 60 seconds. We had a method that received a scope and then checked, if the scope parameter was blank? and aborted the method execution in this case.

Copydef foo(scope)
  return if scope.blank?
  
  # Use scope, e.g.
  scope.find(...)
end

We then called this method with an all scope: foo(Media::Document::Base.all). Be careful with this. all returns a scope (ActiveRecord::Relation) which looks and is harmless at first because nothing is loaded into memory at that point. However, calling .all.blank? loads all records into memory and then evaluates, if the scope is blank.

What you are searching for is a normal nil check, ActiveRecord::FinderMethods.exists? Show archive.org snapshot or its counterpart empty?.

Copydef foo(scope)
  return unless scope
  
  # Use scope, e.g.
  scope.find(...)
end

Copydef foo(scope)
  return unless scope&.exists?
  
  # Use scope, e.g.
  scope.find(...)
end

This will produce an additional query in the database, but is fast and does not load any records into memory.