title: SQLAlchemy ORM load_only colomns with multiple join
body: <p>Now i have:</p>
  
  <pre><code>class DeviceModel(db.Model):
  
      id = db.Column(db.Integer, primary_key=True)
      vendor = db.Column(db.String, nullable=False)
      model = db.Column(db.String, nullable=False)
      group_id = db.Column(db.ForeignKey('device_group.id', ondelete='SET NULL'), nullable=True)
  
  class DeviceGroup(db.Model):
      id = db.Column(db.Integer, primary_key=True)
      name = db.Column(db.String, nullable=False, unique=True)
      device_type_id = db.Column(db.ForeignKey('device_type.id', ondelete='SET NULL'), nullable=True)
      device_type = db.relationship("DeviceType", lazy='joined')
  
  class DeviceType(db.Model):
  
      id = db.Column(db.Integer, primary_key=True)
      name = db.Column(db.String, nullable=False, unique=True)
  </code></pre>
  
  <p>How can i query only <code>DeviceModel.id</code>, <code>DeviceType.name</code> fields?</p>
  
  <p>In pure sql it was:</p>
  
  <pre><code>SELECT device_model.id               AS device_model_id,
         device_type_1.name            AS device_type_1_name
  FROM device_model
         LEFT OUTER JOIN device_group AS device_group_1 ON device_group_1.id = device_model.group_id
         LEFT OUTER JOIN device_type AS device_type_1 ON device_type_1.id = device_group_1.device_type_id;
  </code></pre>