sqlalchemy unique across multiple columns

Question

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\" s

Answer 1

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'),
                     )

Answer 2

from flask_sqlalchemy import SQLAlchemy
db = SQLAlchemy()

class Location(Base):
      __table_args__ = (
        # this can be db.PrimaryKeyConstraint if you want it to be a primary key
        db.UniqueConstraint('customer_id', 'location_code'),
      )
      customer_id = Column(Integer,ForeignKey('customers.customer_id')
      location_code = Column(Unicode(10))