count_sql_for

association_counter.rb

module AssociationCounter
  def count_sql_for(association_sym)
    association_table_name = reflect_on_association(association_sym).klass.table_name
    select("count(#{association_table_name}.id) as #{association_sym}_count").joins(association_sym).to_sql
  end
end
ActiveRecord::Base.send(:extend, AssociationCounter)

example_class.rb

clas Bar < ActiveRecord::Base
  scope :no_foos, lambda{ joins(:foos).where("(#{count_sql_for(:foos)}) = 0") }

end

you don't need to call #to_s inside the string interpolation. It will call it implicitly on a symbol.

Nice catch