class Listing(db.Model):
    id = db.Column(db.Integer, primary_key=True)
title = db.Column(db.String(45), nullable=False)
location_id = db.Column(db.Integer, db.ForeignKey('location.id'))
location = db.relationship('Location', lazy='joined', backref='listings')


class LocationSchema(Schema):
    id = fields.Int()
title = fields.Str()
listings = fields.Nested('ListingSchema', many=True, exclude=('location',))
Serialize two nested schema with marshmallow