Postgres + DataMapper fails to count :id

Gemfile

source :rubygems

gem 'dm-core',          '1.2.0'
gem 'dm-migrations',    '1.2.0'
gem 'dm-timestamps',    '1.2.0'
gem 'dm-validations',   '1.2.0'
gem 'dm-types',         '1.2.0'
gem 'dm-is-tree',       '1.2.0'
gem 'dm-tags',          '1.2.0'
gem 'dm-is-searchable', '1.2.0'
gem 'dm-postgres-adapter'
gem 'dm-mysql-adapter'
gem 'dm-sqlite-adapter'

model.rb

require 'dm-core'
require 'dm-migrations'
require 'test/unit'

DataMapper.setup(:default, 'postgres://test:test@fileserver/test')
# DataMapper.setup(:default, 'mysql://test:test@fileserver/test')
# DataMapper.setup(:default, 'sqlite:test.db')

class Model
  include DataMapper::Resource

  property :id, Serial
  property :name, String
end


class TC_Foo < Test::Unit::TestCase
  def setup
    Model.auto_migrate!
  end

  def test_with_same_id
    rescued = false
    Model.create!(:id => 1, :name => "first record")
    Model.create!(:id => 1, :name => "second record") rescue rescued = true
    assert_true(rescued)
  end

  def test_without_id
    Model.create!(:name => "first record")
    Model.create!(:name => "second record")
  end

  # I expect success, but fail with postgres
  def test_with_id_then_without_id
    Model.create!(:id => 1, :name => "first record")
    Model.create!(:name => "second record")
  end

  def test_with_id_2_then_without_id
    Model.create!(:id => 2, :name => "first record")
    Model.create!(:name => "second record")
  end

  def test_with_different_ids
    Model.create!(:id => 1, :name => "first record")
    Model.create!(:id => 2, :name => "second record")
  end

  def test_without_id_then_with_same_id
    rescued = false
    Model.create!(:name => "first record")
    Model.create!(:id => 1, :name => "second record") rescue rescued = true
    assert_true(rescued)
  end

  def assert_true(actual)
    assert_equal(true, actual)
  end
end