Question

I have a Post model and a View model (which logs the view of each post by a user) as follows:

class View < ActiveRecord::Base
    belongs_to :user, :class_name => 'User'
    belongs_to :post, :class_name => 'Post'
    validates_uniqueness_of :user_id, :scope =>:post_id
end

Each post has a boolean field named “special”.

I want to fetch all posts with following sort conditions:

1. If a post is ‘special’ AND NOT VIEWED by the current user (say user_id=2), then that should appear on top in the result, internally sorted by id in descending order).
2. Sort all other posts (not special and viewed-special) on the basis of id field for the posts.

Cant figure out if and how I can do that.

To start with, I tried the following query:

posts = Post.order("(case when special = true then 'special desc, id desc' else 'id desc' end)")

But it behaves in a funny manner: returns all special posts in top followed by regular posts but with ids sorted in ascending order.

What am i missing here? Also, how can I use join (for views table) with such a complex order by clause? I tried to write a SQL for this but struggling there also! Show me some light 🙂

P.S I’m new to Rails and still learning.

Answer 1

The code you posted is, for each row computing one of 2 string literals and ordering based on that which is why you don’t get the examples you want.
I’d do something like

Post.order('special desc, id desc')

Since rails stores boolean fields as 0/1 in MySQL this will sort special posts ahead of non special posts.

Adding views to this doesn’t change a whole lot: you need to left join the views table and do something like

Post.joins('left join views on posts.id=post_id and user_id=2').order(' ( views.id is null && special)  desc, id desc')

The is null detects whether a view was found which is then and-ed with the post’s specialness.