distinct_relation_for_primary_key

# File activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb, line 1389
      def distinct_relation_for_primary_key(relation) # :nodoc:
        primary_key_columns = Array(relation.primary_key).map do |column|
          visitor.compile(relation.table[column])
        end

        values = columns_for_distinct(
          primary_key_columns,
          relation.order_values
        )

        limited = relation.reselect(values).distinct!
        limited_ids = select_rows(limited.arel, "SQL").map do |results|
          results.last(Array(relation.primary_key).length) # ignores order values for MySQL and PostgreSQL
        end

        if limited_ids.empty?
          relation.none!
        else
          relation.where!(**Array(relation.primary_key).zip(limited_ids.transpose).to_h)
        end

        relation.limit_value = relation.offset_value = nil
        relation
      end