How to achieve rails counter_cache directly in the database.

create_task_trigger.sql

CREATE TRIGGER update_task_counters
  AFTER INSERT OR UPDATE OR DELETE ON tasks
  FOR EACH ROW EXECUTE PROCEDURE update_task_counters();

migration.rb

class AddSomeTriggers < ActiveRecord::Migration
  def change
    execute "DROP TRIGGER IF EXISTS update_user_counters(userid integer);"
    execute %q{
      CREATE OR REPLACE FUNCTION update_user_counters (userid integer)
        RETURNS void AS $$
        BEGIN
          update users set
            tasks_count = coalesce((select count(9) from tasks where user_id = userid and completed = 'f'), 0)
          where id = userid;
        END;
        $$ LANGUAGE plpgsql;
    }
    execute "DROP FUNCTION IF EXISTS update_task_counters;"
    execute %q{
      CREATE OR REPLACE FUNCTION update_task_counters()
        RETURNS trigger AS $update_task_counters$
          BEGIN
            IF (tg_op = 'UPDATE') THEN
              IF new.completed <> old.completed THEN
                PERFORM update_user_counters(new.user_id);
              END IF;
              return new;
            ELSIF (tg_op = 'INSERT') then
              PERFORM update_user_counters(new.user_id);
              RETURN new;
            ELSIF (tg_op = 'DELETE') then
              PERFORM update_user_counters(new.user_id);
              RETURN null;
            END IF;
            RETURN null;
          END;
        $update_task_counters$ LANGUAGE plpgsql;
    }
  execute "DROP TRIGGER IF EXISTS update_task_counters;"
  execute %q{
    CREATE TRIGGER update_task_counters
      AFTER INSERT OR UPDATE OR DELETE ON tasks
      FOR EACH ROW EXECUTE PROCEDURE update_task_counters();
  }
  end
end

update_task_counters.sql

CREATE OR REPLACE FUNCTION update_task_counters()
  RETURNS trigger AS $update_task_counters$
    BEGIN
      IF (tg_op = 'UPDATE') THEN
        IF new.completed <> old.completed THEN
          PERFORM update_user_counters(new.user_id);
        END IF;
        return new;
      ELSIF (tg_op = 'INSERT') then
        PERFORM update_user_counters(new.user_id);
        RETURN new;
      ELSIF (tg_op = 'DELETE') then
        PERFORM update_user_counters(new.user_id);
        RETURN null;
      END IF;
      RETURN null;
    END;
  $update_task_counters$ LANGUAGE plpgsql;

update_user_counters.sql

CREATE OR REPLACE FUNCTION update_user_counters (userid integer)
  RETURNS void AS $$
  BEGIN
    update users set
      tasks_count = coalesce((select count(9) from tasks where user_id = userid and completed = 'f'), 0)
    where id = userid;
  END;
  $$ LANGUAGE plpgsql;