Let's say that I have a class that represents locations. Locations "belong" to customers. Locations are identified by a unicode 10 character code. The "location code" should be unique among the locations for a specific customer.
The two below fields in combination should be unique
customer_id = Column(Integer,ForeignKey('customers.customer_id')
location_code = Column(Unicode(10))
Extract from the documentation of the
unique – When True, indicates that this column contains a unique constraint, or if index is True as well, indicates that the Index should be created with the unique flag. To specify multiple columns in the constraint/index or to specify an explicit name, use the UniqueConstraint or Index constructs explicitly.
As these belong to a Table and not to a mapped Class, one declares those in the table definition, or if using declarative as in the
# version1: table definition mytable = Table('mytable', meta, # ... Column('customer_id', Integer, ForeignKey('customers.customer_id')), Column('location_code', Unicode(10)), UniqueConstraint('customer_id', 'location_code', name='uix_1') ) # or the index, which will ensure uniqueness as well Index('myindex', mytable.c.customer_id, mytable.c.location_code, unique=True) # version2: declarative class Location(Base): __tablename__ = 'locations' id = Column(Integer, primary_key = True) customer_id = Column(Integer, ForeignKey('customers.customer_id'), nullable=False) location_code = Column(Unicode(10), nullable=False) __table_args__ = (UniqueConstraint('customer_id', 'location_code', name='_customer_location_uc'), )