build_relation

# File activerecord/lib/active_record/validations/uniqueness.rb, line 111
      def build_relation(klass, attribute, value)
        relation = klass.unscoped
        comparison = relation.bind_attribute(attribute, value) do |attr, bind|
          return relation.none! if bind.unboundable?

          if !options.key?(:case_sensitive) || bind.nil?
            klass.connection.default_uniqueness_comparison(attr, bind)
          elsif options[:case_sensitive]
            klass.connection.case_sensitive_comparison(attr, bind)
          else
            # will use SQL LOWER function before comparison, unless it detects a case insensitive collation
            klass.connection.case_insensitive_comparison(attr, bind)
          end
        end

        relation.where!(comparison)
      end