title: Serialize two nested schema with marshmallow
body: <p>I am fairly new to python. I have two SQLAlchemy models as follows:</p>
  
  <pre><code>class listing(db.Model):
   id = db.Integer(primary_key=True)
   title = db.String()
   location_id = db.Column(db.Integer, db.ForeignKey('location.id'))
   location = db.relationship('Location', lazy='joined')
  
  class location(db.Model):
   id = db.Integer(primary_key=True)
   title = db.String()
  </code></pre>
  
  <p>I have two Marshmallow schema classes for them:</p>
  
  <pre><code>class ListingSchema(Schema):
   id = fields.Int()
   title = fields.Str()
   location_id = fields.Int()
  
  class LocationSchema(Schema):
   id = fields.Int()
   title = fields.Str()
  </code></pre>
  
  <p>I have created a nested schema class like:</p>
  
  <pre><code>class NestedSchema(Schema):
   listing = fields.Nested(ListingSchema)
   location fields.Nested(LocationSchema)
  </code></pre>
  
  <p>I am doing the join query like :</p>
  
  <pre><code>listing,location = db.session.query(Listing,Location)\
                              .join(Location, and_(Listing.location_id == Location.id))\
                              .filter(Listing.id == listing_id).first()
  </code></pre>
  
  <p>data gets load in the objects i have checked. How can parse this schema?
  I have tried</p>
  
  <pre><code>result,errors = nested_listing_Schema(listing,location)
  </code></pre>
  
  <p>This gives error: "Listing object is not iteratable."</p>