construct_relation_for_exists

# File activerecord/lib/active_record/relation/finder_methods.rb, line 377
      def construct_relation_for_exists(relation, conditions)
        relation = relation.except(:select, :distinct, :order)._select!(ONE_AS_ONE).limit!(1)

        case conditions
        when Array, Hash
          relation.where!(conditions) unless conditions.empty?
        else
          relation.where!(primary_key => conditions) unless conditions == :none
        end

        relation
      end