SQLAlchemy: Any constraint to check one of the two columns is not null?

Question

This may be totally stupid thing to ask but I have such a requirement in my model where atleast either category or parent_category is not null

My model looks like

class BudgetCategories(db.Model):
    __tablename__ = 'budget_categories'
    uuid = Column('uuid', GUID(), default=uuid.uuid4, primary_key=True,
                  unique=True)
    budget_id = Column(GUID(), ForeignKey('budgets.uuid'), nullable=False)
    budget = relationship('Budget', backref='budgetCategories')
    category = Column('category', sa.types.String, nullable=True)
    parent_category = Column('parent_category', sa.types.String, nullable=True)
    amount = Column('amount', Numeric(10, 2), nullable=False)
    recurring = Column('recurring', sa.types.Boolean,
                       nullable=False)
    created_on = Column('created_on', sa.types.DateTime(timezone=True),
                        nullable=False)

How can I specify that. I don't even know what to try

Any pointers appreciated

I am using PostgreSQL as the backend database

Answer 1

I am not 100% sure about the PostgreSQL syntax, but following addition to your BudgetCategories model should do the trick using CheckConstraint:

class BudgetCategories(Base):
    __tablename__ = 'budget_categories'
    # ...

    # @note: new
    __table_args__ = (
            CheckConstraint('NOT(category IS NULL AND parent_category IS NULL)'),
            )

Answer 2

I hope is not too late but this should do the trick and it is checked to work with a PostGreSQL DB:

class BudgetCategories(Base):
    __tablename__ = 'budget_categories'
    __table_args__ = (
        CheckConstraint('coalesce(category , parent_category ) is not null'),
    )
    # ...