CODEWITHSUNDEEP
pythonpostgresqlsqlalchemyflask-sqlalchemy
daydreamer
daydreamer

Reputation: 92149

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

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

Upvotes: 20

Views: 11325

Answers (5)

Robert
Robert

Reputation: 673

I came across this problem some weeks ago, too and was provided with the solution here. So translated to sqlalchemy I'd guess (not yet tested!!):

from sqlalchemy.schema import CheckConstraint

class MyClass(Base):
    __table_args__ = (
        (CheckConstraint(
            'num_nulls(nullif(trim(col_a::text), ''), nullif(trim(col_b::text), '')) = 1', 
            name='uuid_xor_notebook_path')),
    )
    id = Column(Integer, primary_key=True)
    col_a = Column(String, nullable=True, unique=True)
    col_b = Column(String, nullable=True, unique=True)

However for aesthetic reasons I prefer @andilabs solution. To still take into account empty fields for this reason, one could leverage another solution from here and write:

from sqlalchemy.schema import CheckConstraint

class MyClass(Base):
    __table_args__ = (
        (CheckConstraint(
            '(col_a::text IS NULL) <> (col_b IS NULL)', 
            name='uuid_xor_notebook_path')),
    )
    id = Column(Integer, primary_key=True)
    col_a = Column(String, nullable=True, unique=True)
    col_b = Column(String, nullable=True, unique=True)

    @validates('col_a', 'col_b')
    def empty_string_to_null(self, key, value):
        if isinstance(value, str) and value == '':
            return None
        else:
            return value

Not really the shortest of all snippets, I admit...

Upvotes: 0

msh
msh

Reputation: 23

When using the PostgreSQL backend, you can also use the num_nonnulls function for this purpose:

class BudgetCategories(Base):
    __tablename__ = 'budget_categories'
    __table_args__ = (
        CheckConstraint('num_nonnulls(category, parent_category) = 1'),
    )

Upvotes: 2

andilabs
andilabs

Reputation: 23351

I needed XOR behavior in my SQLalchemy models. I come up with the following definition (backend used: PostgreSQL):

from sqlalchemy.schema import (
    CheckConstraint
)

class ScheduledNotebook(Base):
    __table_args__ = (
        (CheckConstraint('(uuid::text IS NULL) <> (notebook_path IS NULL)', name='uuid_xor_notebook_path')),
    )

    id = Column(Integer, primary_key=True)
    notebook_path = Column(String, nullable=True, unique=True)
    uuid = Column(UUID(as_uuid=True), primary_key=True, unique=True, nullable=True)

and following alembic migration (note: autogenerate won't detect it - you have to add it manually):

def upgrade():
    op.create_check_constraint(
        'uuid_xor_notebook_path',
        table_name='scheduled_notebooks',
        schema='metadata',
        condition='(uuid::text IS NULL) <> (notebook_path IS NULL)'
    )


def downgrade():
    op.drop_constraint('uuid_xor_notebook_path')

and it works like a charm:

- only notebook_path - OK

datalake=#  INSERT INTO scheduled_notebooks (schedule,enabled,owner, notebook_path) VALUES ('{"kind":"hourly"}',true,'akos', '/a/b/c/d/e.ipynb');
INSERT 0 1

- only uuid - OK

datalake=#  INSERT INTO scheduled_notebooks (schedule,enabled,owner, uuid) VALUES ('{"kind":"hourly"}',true,'akos', '7792bd5f-5819-45bf-8902-8cf43102434d');
INSERT 0 1

- both uuid and notebook_path - FAILS as desired

datalake=#  INSERT INTO scheduled_notebooks (schedule,enabled,owner, uuid, notebook_path) VALUES ('{"kind":"hourly"}',true,'akos', '7792bd5f-5819-45bf-8902-8cf43102434f', '/a/b/c/d');
ERROR:  new row for relation "scheduled_notebooks" violates check constraint "uuid_xor_notebook_path"
DETAIL:  Failing row contains (567, /a/b/c/d, {"kind": "hourly"}, t, akos, null, null, null, 7792bd5f-5819-45bf-8902-8cf43102434f).

- neither uuid nor notebook_path - FAILS as desired

datalake=#  INSERT INTO scheduled_notebooks (schedule,enabled,owner) VALUES ('{"kind":"hourly"}',true,'akos');
ERROR:  new row for relation "scheduled_notebooks" violates check constraint "uuid_xor_notebook_path"
DETAIL:  Failing row contains (568, null, {"kind": "hourly"}, t, akos, null, null, null, null).

Upvotes: 10

DaniGate
DaniGate

Reputation: 186

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'),
    )
    # ...

Upvotes: 3

van
van

Reputation: 77082

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)'),
            )

Upvotes: 23

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