1259_dm_lh.rb

#!/usr/bin/env ruby
#
# encoding: utf-8

require 'dm-migrations'
require 'dm-validations'

DataMapper::Logger.new $stdout, :debug
DataMapper.setup :default, "sqlite::memory:"

class Tag
  include DataMapper::Resource

  property :id,   Serial
  property :name, String

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

class Entry
  include DataMapper::Resource

  property :id,    Serial
  property :title, String, :length => 255, :index => true

  has n, :entry_tags
  has n, :tags, :through => Resource
end

DataMapper.finalize
DataMapper.auto_migrate!

a = Tag.create( :name => "A" )
b = Tag.create( :name => "B" )
Tag.create( :name => "C" )
Tag.create( :name => "D" )

e = Entry.create(:title => "Hello")
e.tags << a
e.tags << b
e.save

e = Entry.create(:title => "World")
e.tags.create( :name => "H" )
e.tags.create( :name => "I" )
e.tags << a
e.save

# This works:
puts a.entries.size # => 2
puts b.entries.size # => 1

# This blows up now:
puts Entry.all(:tags => a).size # => 2
# ^^^^^^^^^^^
# Tries to run this query: SELECT "id", "title" FROM "entries" WHERE "tag_id" = 1 ORDER BY "id""
puts Entry.all(:tags => b).size # => 1