Rails Advance query CASE WHEN method not working

问题

Is there anyone who can suggest me the solution for a Find_by_sql query here when I am using mysql that work perfect . But when I converted that into active record It is not showing the exact result . First I have tried this code below using find_by_sql:

    @message3 = Message.find_by_sql(["SELECT u.id,c.m_id,u.name,u.email
 FROM messages c, users u
 WHERE (CASE 
 WHEN c.user_one = :id
 THEN c.user_two = u.id
 WHEN c.user_two = #{current_user.id}
 THEN c.user_one= u.id
 END )
 AND (
 c.user_one ='1'
 OR c.user_two ='1'
 )
 Order by c.m_id DESC Limit 20", {:id => current_user.id}])

When I am using this query I am getting the exact value which I want but when I am doing this thing in active record way it is not working well SO here is the code which I have tried in active record style :

@message = Message.select(:user_one, :user_two).distinct.where("user_one = ? OR user_two = ? " , current_user ,current_user)
    @m1 = Message.joins(:user).select("users.id,messages.m_id,users.name,users.email").where("CASE WHEN messages.user_one = #{current_user.id} THEN messages.user_two = users.id WHEN messages.user_two = #{current_user.id} THEN messages.user_one = users.id END").where("messages.user_one = 1 OR messages.user_two = 1 ").order("messages.m_id DESC")

When I am running this query I am getting this generate query :

SELECT users.id,messages.m_id,users.name,users.email FROM `messages` INNER JOIN `users` ON `users`.`id` = `messages`.`user_one` WHERE (CASE WHEN messages.user_one = 1 THEN messages.user_two = users.id WHEN messages.user_two = 1 THEN messages.user_one = users.id END) AND (messages.user_one = 1 OR messages.user_two = 1 )  ORDER BY messages.m_id DESC

Both these query look same but when I am using find_by_sql method I am getting two results but when I am reunnig query of active record it is only showing one when I paste the generated sql in mysql there also I am getting same result IF anyone can help me to understand the difference between these two queries and how can I fix this I mean I need the result of find_by_sql in active record .

回答1:

according to the query your scenario is to pick those messages where either user_one is current user or user_two is the current user. so rather than complex query, what you can simply do is this

Message.where("messages.user_one = ? OR messages.user_two =?", current_user, current_user).order("m_id ASC").limit(20)

now to add detail to it for user. you can either traverse the messages in view and fetch the user for each message and display that. or you can make array of hashes and return that to the view.

来源：https://stackoverflow.com/questions/31736224/rails-advance-query-case-when-method-not-working