title: Datamapper one-to-one association (has 1) issue
body: <p>I am using datamapper in association with postgres</p>
  
  <p>I have 2 classes defined like this:</p>
  
  <pre><code>class Dep
    include DataMapper::Resource
  
    property :id, Serial
    has 1, :rec, 'Rec'
  end 
  
  class Rec
    include DataMapper::Resource  
  
    property :id, Serial
    belongs_to :dep, 'Dep'
  
  end
  </code></pre>
  
  <p>When I try to do so and access the dep model I get an error:
      undefined method 'rec_id' for #</p>
  
  <p>As far as my research goes the model that is created does not have a one to one relationship. Instead it has a one to many relationship. I think that is the problem.
  Is there any way to create a one to one relationship in datamapper?</p>
  
  <p>Further, it does not let me use :required => false with 'has 1' </p>
  
  <p>Using belongs_to both side does not create a proper relationship. It creates to relationships instead which is undesirable.
  Can anyone help me with this issue?</p>