DataMapper generates the wrong SQL query with some relationships.

dm-fail.rb

require "rubygems"
require "datamapper"


class Person
  include DataMapper::Resource

  property :id,     Serial
  property :name,   String, :required => true
  property :role,   String, :set => %w(boss rookie)

  has n, :media, :through => Resource
end

class Medium
  include DataMapper::Resource

  property :id,       Serial
  property :title,     String, :required => true
  property :metadata, Yaml
  
  has n, :people, :through => Resource
  
  def self.filter( f )
    unless f.to_s.strip.empty?
      r = /^#{f.to_s.strip}/i
      rejected = all.map { |m| m.id if (m.title =~ r).nil? && m.metadata && (m.metadata["Keywords"]||[]).grep(r).empty? }.compact
      all(:id.not => rejected)
    else
      all
    end
  end
  
end

DataMapper.finalize
DataMapper::Logger.new(STDOUT,:debug)
DataMapper.setup(:default, "sqlite3::memory:")
DataMapper.auto_migrate!


a = Person.create(:name => "Alice", :role => "boss")
b = Person.create(:name => "Bob", :role => "rookie")

10.times { a.media.create( :title => "XYZ", :metadata => { "Keywords" => %w(one two three four five six)})}
10.times { b.media.create( :title => "ABC", :metadata => { "Keywords" => %w(seven eight nine ten eleven)})}

# Works
p Medium.filter("one")          

# Strangely enough this one works (copied out of Medium#filter):
r = /^seven/i
ids = b.media.map { |m| m.id if (m.title =~ r).nil? && m.metadata && (m.metadata["Keywords"]||[]).grep(r).empty? }.compact
p b.media(:id.not => ids)

# Fails with SQL error: no such column: media.person_id (code: 1, sql state: , query: SELECT "media"."id", "media"."title" FROM "media" INNER JOIN "medium_people" ON "media"."id" = "medium_people"."medium_id" AND "media"."person_id" = 1 INNER JOIN "people" ON "medium_people"."person_id" = "people"."id" WHERE "medium_people"."person_id" = 1 GROUP BY "media"."id", "media"."title" ORDER BY "media"."id", uri: sqlite3://:memory:)
p a.media.filter("eight")