Question

I need to fetch like the winner bids, and a bid can be for a different date (don’t ask why), so I need to select the bid with minimum bid price for each day.

Here are my models

Leilao
  has_many :bids

Bid
  belongs_to :leilao
  #winner_name
  #price
  #date

I tried a solution already and got close to what I need. The problem is that, in some cases, when I create a new bid with lower price, I don’t know why the results do not change.

Leilao.find(1).bids.having('MIN("bids"."price")').group('date').all

This seems to work, but as I said, it does not work in some cases when I create a new bid. But it worked properly once. So, if you do know what might be happening, please tell me.

I then searched for some way for doing this and I got the following

Leilao.find(1).bids.minimum(:price, :group => :date)

which works properly, but with this, I just fetch the dates and prices and I need all the bid data.

I could get it by doing this, but it feels really bad to me

winner_bids = Leilao.find(1).bids.minimum(:price, :group => :date)
winners_data = []
winner_bids.each do |date, price|
  winners_data << Leilao.find(1).bids.where(price: price, date: date).first
end
winners_data

Any idea a better way to do this? Or what’s wrong with my first approach?

Performance is not an issue, since this is just for academic propose but it just feels nasty for me

Also those Leilao.find(1) is just for explaining it here, I’m not using it allover the place, no.

Thanks in advance

Answer 1

see this

mysql> select * from bids;
+----+-------------+-------+------------+-----------+
| id | winner_name | price | date       | leilao_id |
+----+-------------+-------+------------+-----------+
|  1 | A           |   1.1 | 2012-06-01 |         1 |
|  2 | A           |   2.2 | 2012-06-01 |         1 |
|  3 | A           |   3.3 | 2012-05-31 |         1 |
|  4 | A           |   4.4 | 2012-05-31 |         1 |
+----+-------------+-------+------------+-----------+
4 rows in set (0.00 sec)

mysql> select * from bids where leilao_id = 1 group by date order by price asc;
+----+-------------+-------+------------+-----------+
| id | winner_name | price | date       | leilao_id |
+----+-------------+-------+------------+-----------+
|  1 | A           |   1.1 | 2012-06-01 |         1 |
|  3 | A           |   3.3 | 2012-05-31 |         1 |
+----+-------------+-------+------------+-----------+
2 rows in set (0.00 sec)

in rails

1.9.2-p290 :013 > Leilao.find(1).bids.order(:price).group(:date).all
[
    [0] #<Bid:0x00000003553838> {
                 :id => 1,
        :winner_name => "A",
              :price => 1.1,
               :date => Fri, 01 Jun 2012,
          :leilao_id => 1
    },
    [1] #<Bid:0x00000003553518> {
                 :id => 3,
        :winner_name => "A",
              :price => 3.3,
               :date => Thu, 31 May 2012,
          :leilao_id => 1
    }
]