Bleeding Fingers Bleeding Fingers - 29 days ago 4
Python Question

SQLAlchemy not producing proper SQL statement for multi column UniqueConstraints

Below are the two different attempts I have made in trying to achieve a multi-column unique constraint in sqlalchemy, both of which seems to have failed since a proper SQL statement is not being produced.

The attempts:

from sqlalchemy import Column, Integer, String, Text, ForeignKey, DateTime, create_engine, UniqueConstraint, Boolean
from sqlalchemy.orm import relationship, backref, sessionmaker
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.interfaces import PoolListener
import sqlalchemy

class ForeignKeysListener(PoolListener):
def connect(self, dbapi_con, con_record):
db_cursor = dbapi_con.execute('pragma foreign_keys=ON')

engine = create_engine(r"sqlite:///" + r"d:\\foo.db",
listeners=[ForeignKeysListener()], echo = True)
Session = sessionmaker(bind = engine)
ses = Session()
Base = declarative_base()
print sqlalchemy.__version__
class Foo(Base):
__tablename__ = "foo"

id = Column(Integer, primary_key=True)
dummy = Column(Integer, unique = True)
class Bar(Base):
__tablename__ = "bar"
id = Column(Integer, primary_key=True)
baz = Column(Integer, ForeignKey("foo.id"))
qux = Column(Integer, ForeignKey("foo.id"))
UniqueConstraint("baz", "qux")

class Cruft(Base):
__tablename__ = "cruft"

id = Column(Integer, primary_key=True)
bar = Column(Integer, ForeignKey("foo.id"))
qux = Column(Integer, ForeignKey("foo.id"))
__table_args = (UniqueConstraint("bar", "qux"),)
Base.metadata.create_all(engine)


The output:

>>> 0.8.2
2013-05-09 16:25:42,677 INFO sqlalchemy.engine.base.Engine PRAGMA table_info("foo")
2013-05-09 16:25:42,677 INFO sqlalchemy.engine.base.Engine ()
2013-05-09 16:25:42,677 INFO sqlalchemy.engine.base.Engine PRAGMA table_info("bar")
2013-05-09 16:25:42,677 INFO sqlalchemy.engine.base.Engine ()
2013-05-09 16:25:42,677 INFO sqlalchemy.engine.base.Engine PRAGMA table_info("cruft")
2013-05-09 16:25:42,677 INFO sqlalchemy.engine.base.Engine ()
2013-05-09 16:25:42,677 INFO sqlalchemy.engine.base.Engine
CREATE TABLE foo (
id INTEGER NOT NULL,
dummy INTEGER,
PRIMARY KEY (id),
UNIQUE (dummy)
)


2013-05-09 16:25:42,677 INFO sqlalchemy.engine.base.Engine ()
2013-05-09 16:25:42,767 INFO sqlalchemy.engine.base.Engine COMMIT
2013-05-09 16:25:42,769 INFO sqlalchemy.engine.base.Engine
CREATE TABLE bar (
id INTEGER NOT NULL,
baz INTEGER,
qux INTEGER,
PRIMARY KEY (id),
FOREIGN KEY(baz) REFERENCES foo (id),
FOREIGN KEY(qux) REFERENCES foo (id)
)


2013-05-09 16:25:42,769 INFO sqlalchemy.engine.base.Engine ()
2013-05-09 16:25:42,838 INFO sqlalchemy.engine.base.Engine COMMIT
2013-05-09 16:25:42,839 INFO sqlalchemy.engine.base.Engine
CREATE TABLE cruft (
id INTEGER NOT NULL,
bar INTEGER,
qux INTEGER,
PRIMARY KEY (id),
FOREIGN KEY(bar) REFERENCES foo (id),
FOREIGN KEY(qux) REFERENCES foo (id)
)


2013-05-09 16:25:42,839 INFO sqlalchemy.engine.base.Engine ()
2013-05-09 16:25:42,917 INFO sqlalchemy.engine.base.Engine COMMIT


Any suggestions?

Answer

When using a UniqueConstraint in a declarative table configuration, you need to specify it using the __table_args__ attribute (note the underscores on both sides of the name:

class Bar(Base):
    __tablename__ = "bar"
    __table_args__ = (UniqueConstraint("baz", "qux"),)

    id = Column(Integer, primary_key=True)
    baz = Column(Integer, ForeignKey("foo.id"))
    qux = Column(Integer, ForeignKey("foo.id"))

class Cruft(Base):
    __tablename__ = "cruft"
    __table_args__ = (UniqueConstraint("bar", "qux"),)

    id = Column(Integer, primary_key=True)
    bar = Column(Integer, ForeignKey("foo.id"))
    qux = Column(Integer, ForeignKey("foo.id"))

The attribute must be either a tuple or a dictionary.

Creating these two tables now results in:

2013-05-09 13:38:44,180 INFO sqlalchemy.engine.base.Engine 
CREATE TABLE cruft (
    id INTEGER NOT NULL, 
    bar INTEGER, 
    qux INTEGER, 
    PRIMARY KEY (id), 
    UNIQUE (bar, qux), 
    FOREIGN KEY(bar) REFERENCES foo (id), 
    FOREIGN KEY(qux) REFERENCES foo (id)
)

...

2013-05-09 13:38:44,181 INFO sqlalchemy.engine.base.Engine 
CREATE TABLE bar (
    id INTEGER NOT NULL, 
    baz INTEGER, 
    qux INTEGER, 
    PRIMARY KEY (id), 
    UNIQUE (baz, qux), 
    FOREIGN KEY(baz) REFERENCES foo (id), 
    FOREIGN KEY(qux) REFERENCES foo (id)
)

Also see the Setting up Constraints when using the Declarative ORM Extension section of the Defining Constraints and Indexes chapter.

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