Environment details

OS: Windows 10
Programming language: Python
Language runtime version: 3.9.12
sqlalchemy-spanner version: 1.2.1
google-cloud-spanner version: 3.17.0
alembic version : 1.8.1

Steps to reproduce

Add a 'unique' constraint to a model thus:

class Tag(Base):
    __tablename__ = 'tag'
    tag = Column(String, primary_key=True)
    test = Column(String, unique=True)

Output

Whether this line is added to an existing model or when creating a new one, the line generated for the migration is either op.create_unique_constraint(None, ['test']) in an op.batch_alter_table(), or sa.UniqueConstraint('version') in an op.create_table().

Running either of those during a migrations gives the following error:

google.cloud.spanner_dbapi.exceptions.ProgrammingError: Spanner doesn't support direct UNIQUE constraints creation. Create UNIQUE indexes instead.