how do you define types if you custom select in a rails query

My query looks like this:

@posts = Post.includes(:last_comment)
    .joins("LEFT OUTER JOIN read_marks ON posts.id = read_marks.post_id AND read_marks.user_id = #{user.id}")
    .where(:postr_id => postr.id)
    .select("post开发者_如何学Pythons.*, read_marks.comments_cache as rm_comments_cache, read_marks.timestamp as last_read_at")

But when I call @posts.each{|post| post.last_read_at} it returns a string and not a datetime.

Why don't you try using the ReadMark model?

@posts = Post.includes(:read_marks)
    .joins("LEFT OUTER JOIN read_marks ON posts.id = read_marks.post_id and read_marks.user_id = #{user.id}")
    .where(:postr_id => postr.id)

@posts.each{|post| post.read_marks.timestamp}

This method is cleaner, and uses ActiveRecord in the way it was designed.

If you really want to use your original query, you can parse the date manually.

@posts.each{|post| Date.parse post.last_read_at }

Even though last_read_at should be a datetime ActiveRecord does not automatically deserialize non-column values. You'll have to parse the datetime yourself.

@posts.each{|post| post.last_read_at = Time.zone.parse(post.last_read_at)}