How to fetch distinct values with arel/relational algebra and has_many :through

Question

When I try to display all movies that a person is in, and they have more than 1 role (director,writer,actor) in a movie, I get multiple lines for that movie. If I add .select('DISTINCT id') or movies.* to try and eliminate the dups I get the following error:

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 'DISTINCT id FROM movies INNER JOIN movie_people ON movies.id = movie_peop' at line 1: SELECTmovies.*, DISTINCT id FROMmoviesINNER JOINmovie_peopleONmovies.id =movie_people.movie_id WHERE ((movie_people`.person_id = 601619)) ORDER BY title LIMIT 18 OFFSET 0

I don't know how to code the arel request correctly. Please help.
Thank you.

app/controllers

class PeopleController < ApplicationController  
def show  
    @person = Person.find(params[:id])  
    @movies = @person.movies.select('DISTINCT id').
      paginate :per_page => 18, :page => params[:page],
               :order => sort_order(params[:sort]) 
end  

app/models

class Person < ActiveRecord::Base  
  has_many :movie_people  
  has_many :movies, :through => :movie_people  
end  

class MoviePerson < ActiveRecord::Base  
  belongs_to :movie  
  belongs_to :person  
end  

class Movie < ActiveRecord::Base  
  has_many :movie_people  
  has_many :people, :through => :movie_people  
end  

db/schema.rb

  create_table "people", :force => true do |t|  
    t.string   "name"  
  end  

  create_table "movie_people", :force => true do |t|  
    t.integer  "movie_id"  
    t.integer  "person_id"  
    t.integer  "role"  
  end  

  create_table "movies", :force => true do |t|  
    t.string   "title"  
    t.string   "year"  
  end  

movies/show.html.erb

Title:<%= @movie.title %><br>
Year:<%= @movie.year %><br>
<% @movie.movie_people.group_by(&:role).each do |r, a| %>  
 <%= %w(Director: Writer: Cast:)[r] %>  
  <% a.each do |c| %>  
   <%= link_to c.person.name, 
       :controller => 'people', :action => 'show', :id => c.person.id %>
  <% end %><br>  
<% end %>  

Title: Fahrenheit 9/11
Year: 2004
Director: Michael Moore
Writer: Michael Moore
Cast: Michael Moore George W. Bush

people/show.html.erb

Name:<%= @person.name %>
<% @movies.each do |movie| %>  
<br><%= link_to movie.title, movie %> (<%= movie.year %>)  
<% end %>  

Name: Michael Moore
Fahrenheit 9/11 (2004)
Fahrenheit 9/11 (2004)
Fahrenheit 9/11 (2004)

Answer 1

In Arel, uniq generates the appropriate select distinct SQL. I'm not sure how to add Arel to a has_many association, but in regular (controller) code it would be something like

@person.movies.uniq

See http://apidock.com/rails/ActiveRecord/QueryMethods/uniq and Rails Arel selecting distinct columns

Answer 2

This is probably a bug with Rails.

In the meantime, you can exclude the select before select distinct, like so:

@person.movies.except(:select).select('DISTINCT id')