arel union and latest conversation from messages - mysql

Question

I need a list of messages where each one is the most recent in the "conversation" between the current user and each other user.
The same query is described in this question
The code I have so far is:
t1 = Arel::Table.new(:messages, :as => 't1')
t2 = Arel::Table.new(:messages, :as => 't2')
convs1 = t1.
project(
t1[:receiver_user_id].as('other_user_id'),
t1[:receiver_user_id].as('receiver_user_id'),
t1[:sender_user_id].as('sender_user_id'),
t1[:created_at].as('created_at')
).
where(t1[:sender_user_id].eq(user.id))
convs2 = t2.project(
t2[:sender_user_id].as('other_user_id'),
t2[:receiver_user_id].as('receiver_user_id'),
t2[:sender_user_id].as('sender_user_id'),
t2[:created_at].as('created_at')
).
where(t2[:receiver_user_id].eq(user.id))
conv = convs1.union(convs2)
First off, I get an error:
ActiveRecord::StatementInvalid: Mysql2::Error: You have an error in your SQL syntax; check \
the manual that corresponds to your MySQL server version for the right syntax to use near \
'UNION SELECT `t2`...
This works if I manually replace "UNION" with "UNION ALL" in the sql produced below.
conv.to_sql from the above code produces:
SELECT `t1`.`receiver_user_id` AS other_user_id,
`t1`.`receiver_user_id` AS receiver_user_id, `
t1`.`sender_user_id` AS sender_user_id,
`t1`.`created_at` AS created_at
FROM `messages` `t1`
WHERE `t1`.`sender_user_id` = 50
UNION
SELECT `t2`.`sender_user_id` AS other_user_id,
`t2`.`receiver_user_id` AS receiver_user_id,
`t2`.`sender_user_id` AS sender_user_id,
`t2`.`created_at` AS created_at
FROM `messages` `t2`
WHERE `t2`.`receiver_user_id` = 50
Any idea why there's a MySQL UNION error. Is it an arel bug?
Secondly, any help with completing the query would be much appreciated.
Update:
Using Arel::Nodes::Union.new works

Answer 1

I think this is more probably a mysql fault, this is a mySQL error text. Something similar is discussed in here, but not exactly this issue.
Try to migrate to another sql server, and check again, or if union all works then use this:
conv = convs1.union(convs2, :all)
Based on documentation.

Answer 2

The problem is actually the parentheses in the sql. It works if I run:
Message.find_by_sql conv.to_sql.delete('()')
which removes the leading "(" and trailing ")"
Weird.. I don't know how I would chain this to complete the query. (Arel::Nodes::Union doesn't have a group method). This is Rails 3.1.4

Answer 3

I had a similar problem and solved it as follows:
def last_messages
Message.find_by_sql("
SELECT messages.*,
(IF(recipient_id = #{id}, 0,1)) as outlast,
users.avatar_name,
users.name
FROM messages
INNER JOIN users
ON users.id=(IF(recipient_id = #{id}, sender_id,recipient_id))
WHERE messages.id IN
( SELECT max(id)
FROM messages
WHERE recipient_id = #{id} OR sender_id = #{id}
GROUP BY (IF(recipient_id = #{id}, sender_id, recipient_id))
)
ORDER BY messages.id DESC")
end

Answer 4

This is the code I used instead in the end
all_msgs = Message.where("messages.sender_user_id = ? OR messages.receiver_user_id = ?",
user.id, user.id)
msg_ids = all_msgs.select("sender_user_id, receiver_user_id, max(id) as max_id")
.group(:sender_user_id, :receiver_user_id).map { |m| m.max_id }
all_msgs = all_msgs.where(:id => msg_ids)