Rails, SQL: private chat, how to find last message in each conversation - sql

Question

I'v got the folowing schema
+----+------+------+-----------+---------------------+--------+
| id | from | to | message | timestamp | readed |
+----+------+------+-----------+---------------------+--------+
| 46 | 2 | 6 | 123 | 2013-11-19 19:12:19 | 0 |
| 44 | 2 | 3 | 123 | 2013-11-19 19:12:12 | 0 |
| 43 | 2 | 1 | ????????? | 2013-11-19 18:37:11 | 0 |
| 42 | 1 | 2 | adf | 2013-11-19 18:37:05 | 0 |
+----+------+------+-----------+---------------------+--------+
from/to is the ID of the user's, message – obviously, the message, timestamp and read flag.
When user open's his profile I want him to see the list of dialogs he participated with last message in this dialog.
To find a conversation between 2 people I wrote this code, it's simple (Message model):
def self.conversation(from, to)
where(from: [from, to], to: [from, to])
end
So, I can now sort the messages and get the last one. But it's not cool to fire a lot of queries for each dialog.
How could I achieve the result I'm looking for with less queries?
UPDATE:
Ok, looks like it's not really clear, what I'm trying to achieve.
For example, 4 users – Kitty, Dandy, Beggy and Brucy used that chat.
When Brucy entered in dialogs, she shall see
Beggy: hello brucy haw ar u! | <--- the last message from beggy
-------
Dandy: Hi brucy! | <---- the last message from dandy
--------
Kitty: Hi Kitty, my name is Brucy! | <–– this last message is from current user
So, three separated dialogs. Then, Brucy can enter anyone dialog to continue private conversation.
And I can't figured out how could I fetch this records without firing a query for each dialog between users.

Answer 1

This answer is a bit late, but there doesn't seem to be a great way to do this, in Rails 3.2.x at least.
However, here is the solution I came up with
(as I had the same problem on my website).
#sender_ids =
Message.where(recipient_id: current_user.id)
.order("created_at DESC")
.select("DISTINCT owner_id")
.paginate(per_page: 10, page: params[:page])
sql_queries =
#sender_ids.map do |user|
user_id = user.owner_id
"(SELECT * FROM messages WHERE owner_id = #{user_id} "\
"AND recipient_id = #{current_user.id} ORDER BY id DESC "\
"LIMIT 1)"
end.join(" UNION ALL ")
#messages = Message.find_by_sql(sql_queries)
ActiveRecord::Associations::Preloader.new(#messages, :owner).run
This gets the last 10 unique people you sent messages to.
For each of those people, it creates a UNION ALL query to get the last message sent to each of those 10 unique people. With 50, 000 rows, the query completes in about ~20ms. And of course to get assocations to preload, you have to use .includes will not work when using .find_by_sql

Answer 2

def self.conversation(from, to)
order("timestamp asc").last
end
Edit:
This railscast will be helpful..
http://railscasts.com/episodes/316-private-pub?view=asciicast
EDIT2:
def self.conversation(from, to)
select(:from, :to, :message).where(from: [from, to], to: [from, to]).group(:from, :to, :country).order("timestamp DESC").limit(1)
end