Python – sqlalchethe unique across multiple columns

pythonsqlalchemy

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))

So if i have two customers, customer "123" and customer "456". They both can have a location called "main" but neither could have two locations called main.

I can handle this in the business logic but I want to make sure there is no way to easily add the requirement in sqlalchemy. The unique=True option seems to only work when applied to a specific field and it would cause the entire table to only have a unique code for all locations.

Best Answer

Extract from the documentation of the Column:

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 __table_args__:

# 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'),
                     )
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