Equivalent in MySQL

# returns duplicates, essentially the same as the datamapper query
SELECT books.*
FROM books
INNER JOIN readers ON readers.book_id = books.id
WHERE readers.user_id = xxx OR books.private = FALSE

# gets rid of the duplicates because of the DISTINCT
SELECT DISTINCT books.*
FROM books
INNER JOIN readers ON readers.book_id = books.id
WHERE readers.user_id = xxx OR books.private = FALSE

gistfile2.rb

#!/usr/bin/env ruby -KU

require 'rubygems'
require 'do_sqlite3'
require 'dm-core'
require 'pp'

DataMapper.logger = DataMapper::Logger.new($stderr, :debug)
DataMapper.setup(:default, 'sqlite3://memory')

class User
  include DataMapper::Resource
  
  property :id,   Serial
  property :name, String
  
  has n, :readers
  has n, :books, :through => :readers
end

class Reader
  include DataMapper::Resource

  belongs_to :user
  belongs_to :book
  
  property :id,   Serial
  property :book_id, Integer  
  property :user_id, Integer
end

class Book
  include DataMapper::Resource
  
  property :id,   Serial
  property :name, String
  property :private, Boolean, :default => true
  
  has n, :readers
  
  def self.accessible_by(user)
    all(
      :links      => [Book.relationships[:readers]],
      :conditions => ["(readers.user_id = ? OR books.private = ?)", user.id, false]      
    )
  end
end

DataMapper.auto_migrate!

tom   = User.create(:name => "tom")
dick  = User.create(:name => "dick")
harry = User.create(:name => "harry")

public_book   = Book.create(:name => "public", :private => false)
private_book  = Book.create(:name => "private")

Reader.create(:user => tom,   :book => private_book)
Reader.create(:user => dick,  :book => public_book)
Reader.create(:user => harry, :book => public_book)

pp Book.accessible_by(tom) 

# This outputs the following:
# =>  [#<Book id=1 name="public" private=false>, #<Book id=1 name="public" private=false>, #<Book id=2 name="private" private=true>]

# Obviously I want just:
# =>  [#<Book id=1 name="public" private=false>, #<Book id=2 name="private" private=true>]

The answer

class Book
  include DataMapper::Resource
  
  property :id,   Serial
  property :name, String
  property :private, Boolean, :default => true
  
  has n, :readers
  
  def self.accessible_by(user)
    all(
      :links      => [Book.relationships[:readers]],
      :conditions => ["(readers.user_id = ? OR books.private = ?)", user.id, false],
      # add this!
      :unique     => true  
    )
  end
end