title: ObjectModel accessing incoming relations
body: <p>I have two nodes <code>A</code> and <code>B</code>. They have a directed relation from <code>A</code> to <code>B</code>. </p>
  
  <p>Thus, <code>A</code> has a <code>ConnectedTo</code> attributed of type <code>RelatedTo</code>. However, I want to iterate over all <code>B</code> nodes and access the incoming relations from <code>A</code>.</p>
  
  <p><strong>How can I do this?</strong></p>
  
  <p>I tried adding a <code>ConnectedTo</code> attribute of type <code>RelatedFrom</code> to <code>B</code> but when querying the graph I get a <code>ValueError('Invalid Identifier')</code>.</p>
  
  <pre><code>class A(GraphObject):
  
      __primarykey__ = "hash"
  
      hash = Property()
  
      ConnectedTo = RelatedTo('B')
  
      def __init__(self, hash):
          self.hash = hash
  
  
  class B(GraphObject):
  
      __primarykey__ = "hash"
  
      hash = Property()
  
      ConnectedTo = RelatedFrom('A')
  
      def __init__(self, hash):
          self.hash = hash
  
  
  &gt;&gt;&gt; a = A("testA")
  &gt;&gt;&gt; b = B("testB")
  &gt;&gt;&gt; a.ConnectedTo.add(b)
  &gt;&gt;&gt; graph.push(a)
  &gt;&gt;&gt; graph.push(b)
  &gt;&gt;&gt; test = B.select(graph).first()
  </code></pre>
  
  <p>Results in error:</p>
  
  <pre><code>Traceback (most recent call last):
    File "&lt;stdin&gt;", line 1, in &lt;module&gt;
    File "/usr/local/lib/python2.7/dist-packages/py2neo/ogm.py", line 442, in first
      return self._object_class.wrap(super(GraphObjectSelection, self).first())
    File "/usr/local/lib/python2.7/dist-packages/py2neo/ogm.py", line 344, in wrap
      _ = getattr(inst, attr)
    File "/usr/local/lib/python2.7/dist-packages/py2neo/ogm.py", line 90, in __get__
      related[key] = RelatedObjects(cog.node, self.direction, self.relationship_type, self.related_class)
    File "/usr/local/lib/python2.7/dist-packages/py2neo/ogm.py", line 135, in __init__
      self.__relationship_pattern = "(a)&lt;-[_:%s]-(b)" % cypher_escape(relationship_type)
    File "/usr/local/lib/python2.7/dist-packages/py2neo/database/cypher.py", line 221, in cypher_escape
      writer.write_identifier(identifier)
    File "/usr/local/lib/python2.7/dist-packages/py2neo/database/cypher.py", line 78, in write_identifier
      raise ValueError("Invalid identifier")
  ValueError: Invalid identifier
  </code></pre>