title: Filter SQLAlchemy query result object's one-to-many attribute
body: <p>Say I have a couple objects, having a one-to-many relationship, something like</p>
  
  <pre><code>class Parent():
      //id, other cols, etc
      children = relationship("Child", backref="parent")
  
  class Child():
      parent_id = Column(Integer, ForeignKey("parent.id")
      child_type = Column(Enum("a","b"))
  </code></pre>
  
  <p>Now, I want to query Parent objects, but have their children filtered by child_type, ie something like</p>
  
  <pre><code>session.query(Parent).join(Parent.children).filter(Child.child_type == "a")
  </code></pre>
  
  <p>This just returns the Parent with all the children, basically disregarding the filter. Is this result at all possible or do I have to also query Child?</p>