Select minimum value from a group in activerecord - ruby-on-rails

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
}
]

Answer 2

As Amol said its not going to work, the SQL looks like
SELECT "whatever".* FROM "whatever" GROUP BY date ORDER BY price
But the group_by will be applied before order_by
I had this trouble when i had market table where is stored different types of records, i solved it by writing a class method, maybe not the best approach but working
def self.zobraz_trh(user)
markets = []
area = self.group(:area).all.map &:area
area.each do |market|
markets << self.where(["area = ? and user_id != ?", market,user]).order(:price).first
end
markets
end
where self is Market class