Question

I have a Department model with two associations to User such that

class Department < ActiveRecord::Base
   has_many :members, class_name:"User", foreign_key:"department_id"
   belongs_to :director, class_name:"User", foreign_key:"director_id
end

To display this model in a listing, sorted by number of members I can do this

@departments = Department.select("departments.*, COUNT(users.id) AS members").joins(:members).group("departments.id")
@departments = @departments.order("members ASC")

If I wanted to sort by a field (last_name) in the director’s associated model, I could do this:

@departments = Department.includes(:director)
@departments = @departments.order("users.last_name")

However, I can’t combine these two queries into one that allows correct sorting of both. If I do

Department.select("departments.*, COUNT(users.id) AS members").joins(:members, :director).group("departments.id, users.last_name")

then I get a vast ammount of duplicate results, and I also exclude resuls where director_id is nil. How can I structure the query so that I can order by both COUNT(users.id) and essentially director.last_name ?

Edit: This is the .to_sql for that bad combined query:

SELECT departments.*, COUNT(users.id) AS members FROM "departments" 
INNER JOIN "users" ON "users"."department_id" = "departments"."id" 
INNER JOIN "users" "directors_departments"    
ON "directors_departments"."id" = "departments"."director_id" 
GROUP BY departments.id, users.last_name 
ORDER BY users.last_name DESC

Answer 1

So there are 2 problems here:

1. You are using .joins on a belongs_to association that may not exist.
2. You are joining the same user table twice (for 2 different associations), causing rails to generate a table alias so that the SQL will be valid. But you aren’t grouping on this alias.

Solution:

You’ll need a LEFT JOIN which unfortunately isn’t as pretty as a standard join. You’ll also need to modify your .group clause:

Department.
  select("departments.*, COUNT(users.id) AS members").
  joins(:members, "LEFT JOIN users directors ON departments.director_id = directors.id").
  group("departments.id, directors.last_name").
  order("members, directors.last_name")