title: manytomany fields not visible in _meta.fields
body: <p>I have a generic function that iterates over _meta.fields of a given object. All field names and values are fetched correctly except for ManyToMany fields. It seems to completely ignore ManyToMany fields. How do we retrive the fks from m2m fields?</p>
  
  <pre><code>def myfunc(self)
      for field in self._meta.fields:
          type = field.get_internal_type()
          name = field.name
          val = getattr(self,field.name)
  </code></pre>