SQLAlchemy one to many relationship, how to filter the collection

Question

Bacially Product has a one to many relationship to ProductPicture.

My product picture model looks like this:

picture_type_enums = ('main', 'related', 'option')

class ProductPicture(Base):

    __tablename__ = 'product_pictures'

    picture_id = Column(Integer, primary_key = True)
    product_id = Column(Integer, ForeignKey('products.product_id'))
    picture_type = Column(Enum(*picture_type_enums))
    url = Column(String(120))

and my product model looks like this:

class Product(Base):

    __tablename__ = 'products'

    product_id = Column(Integer, primary_key=True)
    product_name = Column(String(100))
    product_pictures = relationship("ProductPicture")

My question is for one product I might have different types of product pictures. I know if I have a Product instance p, I can call p.product_pictures to get all types of product pictures. However, I want something like p.main_pictures which get all the product pictures of type 'main', and p.option_pictures gets all the product_pictures of type 'option'. Is there a good way to do it.

Thanks

Answer 1

If you read the documentation for sqlalchemy.orm.relationship, you can see that you can further limit the relationship by explicitly define the condition using the primaryjoin argument, with an example that perfectly illustrating your required scenario. Adapting that with your requirements, the Product class now follows:

class Product(Base):

    __tablename__ = 'products'

    product_id = Column(Integer, primary_key=True)
    product_name = Column(String(100))
    product_pictures = relationship("ProductPicture")

    main_pictures = relationship("ProductPicture",
        primaryjoin="and_(Product.product_id==ProductPicture.product_id, "
                    "ProductPicture.picture_type=='main')")
    option_pictures = relationship("ProductPicture",
        primaryjoin="and_(Product.product_id==ProductPicture.product_id, "
                    "ProductPicture.picture_type=='option')")

Example session:

>>> p = Product()
>>> p.product_name = 'test product'
>>> p.product_id = 1
>>> session.add(p)
>>> pic1 = ProductPicture()
>>> pic1.product_id = p.product_id
>>> pic1.picture_type = 'main'
>>> pic1.url = 'http://example.com/p1.main.png'
>>> session.add(pic1)
>>> pic2 = ProductPicture()
>>> pic2.product_id = p.product_id
>>> pic2.picture_type = 'option'
>>> pic2.url = 'http://example.com/p1.option1.png'
>>> session.add(pic2)
>>> pic3 = ProductPicture()
>>> pic3.product_id = p.product_id
>>> pic3.picture_type = 'option'
>>> pic3.url = 'http://example.com/p1.option2.png'
>>> session.add(pic3)
>>> session.commit()
>>> [(pic.picture_type, pic.url) for pic in p.product_pictures]
[(u'main', u'http://example.com/p1.main.png'), (u'option', u'http://example.com/p1.option1.png'), (u'option', u'http://example.com/p1.option2.png')]
>>> [(pic.picture_type, pic.url) for pic in p.main_pictures]
[(u'main', u'http://example.com/p1.main.png')]
>>> [(pic.picture_type, pic.url) for pic in p.option_pictures]
[(u'option', u'http://example.com/p1.option1.png'), (u'option', u'http://example.com/p1.option2.png')]