dm_issue_1367.rb

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

require 'dm-sqlite-adapter'
require 'dm-migrations'
require 'dm-validations'

DataMapper.setup :default, "sqlite::memory:"

class Ban
  include DataMapper::Resource

  property :id, Serial

  belongs_to :user # required by default
  has n, :users, :through => Resource
end

class User
  include DataMapper::Resource

  property :id, Serial
end

DataMapper.finalize
DataMapper.auto_migrate!

b = Ban.new
puts b.valid?
puts b.errors[:user_id].inspect

dm_issue_1367_output.txt

false
["User must not be blank"]