module ActiveRecord
  module ConnectionAdapters
    class PostgreSQLAdapter
      def pk_and_sequence_for(table)
        # First try looking for a sequence with a dependency on the
        # given table's primary key.
        result = query(<<-end_sql, 'PK and serial sequence')[0]
          SELECT attr.attname, seq.relname
          FROM pg_class      seq,
               pg_attribute  attr,
               pg_depend     dep,
               pg_namespace  name,
               pg_constraint cons
          WHERE seq.oid           = dep.objid
            AND seq.relkind       = 'S'
            AND attr.attrelid     = dep.refobjid
            AND attr.attnum       = dep.refobjsubid
            AND attr.attrelid     = cons.conrelid
            AND attr.attnum       = cons.conkey[1]
            AND cons.contype      = 'p'
            AND dep.classid       = '"pg_class"'::regclass
            AND dep.refclassid    = '"pg_class"'::regclass
            AND dep.refobjid      = '#{quote_table_name(table)}'::regclass
        end_sql

        if result.nil? or result.empty?
          # If that fails, try parsing the primary key's default value.
          # Support the 7.x and 8.0 nextval('foo'::text) as well as
          # the 8.1+ nextval('foo'::regclass).
          result = query(<<-end_sql, 'PK and custom sequence')[0]
            SELECT attr.attname,
              CASE
                WHEN split_part(def.adsrc, '''', 2) ~ '.' THEN
                  substr(split_part(def.adsrc, '''', 2),
                         strpos(split_part(def.adsrc, '''', 2), '.')+1)
                ELSE split_part(def.adsrc, '''', 2)
              END
            FROM pg_class       t
            JOIN pg_attribute   attr ON (t.oid = attrelid)
            JOIN pg_attrdef     def  ON (adrelid = attrelid AND adnum = attnum)
            JOIN pg_constraint  cons ON (conrelid = adrelid AND adnum = conkey[1])
            WHERE t.oid = '#{quote_table_name(table)}'::regclass
              AND cons.contype = 'p'
              AND def.adsrc ~* 'nextval'
          end_sql
        end

        # [primary_key, sequence]
        [result.first, result.last]
      rescue
        nil
      end
    end
  end
end