Question

I need to write two classes like this:

class Item(Base, DBBase):
    __tablename__ = 'items'

    id = Column(Integer, primary_key = True)
    name = Column(String)
    description = Column(String)
    price = Column(Float, default = 0)
    on_sell = Column(Boolean, default = False)

    img = Column(String)

    attributes = relationship('ItemAttribute')

    def __init__(self, name, description):
        self.name = name
        self.description = description

class ItemAttribute(Base, DBBase):
    __tablename__ = 'itemattributes'

    id = Column(Integer, primary_key = True)
    name = Column(String, nullable = False)
    value = Column(String, nullable = False)

    item_id = Column(Integer, ForeignKey('items.id'))
    item = relationship('Item')

    def __init__(self, name, value):
        self.name = name
        self.value = value

One item can own several attributes, and I need to:
1. insert some methods on class Item to easily do CURD(insertion, deletion, update and query) attributes for it. I need to search a attribute of a item and return it’s corresponding value.
2. have the ability to search items by attributes. For example, some items have the attributes of ‘Feature’ = ‘True’. I need to get all items which have this attribute.

Thanks for help. 🙂

Answer 1

If you add backref onto your ItemAttribute relationship:

item_id = Column(Integer, ForeignKey('items.id', onupdate='CASCADE', ondelete='CASCADE'))
item = relationship(Items, backref='attributes')

This will create and Item.attributes[] array which contains the ItemAttribute’s. You might also add the onupdate and ondelete if you’re using mysql.

Then when you query, you can do this:

rs = mySession.query(Items)
firstItem = rs.first()
for attribute in firstItem.attributes:
   print attribute

When querying you can filter by joining the backref:

rs = mySession.query(Items).join(Items.attributes).filter(ItemAttribute.name=='somethingSpecial')

Further, if it’s a one to one relationship (but it’s not in this case), you could skip the list by specifing uselist=False:

item = relationship(ITEM, backref='attribute', uselist=False)