SQLAlchemy: how exchange unique values in one transaction? - mysql

Question

My versions:
$ python
Python 3.4.0 (default, Apr 11 2014, 13:05:11)
[GCC 4.8.2] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import sqlalchemy
>>> sqlalchemy.__version__
'1.0.0'
Let's look to the example for sqlite (in production I use mysql, but it does not matter here):
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker
from sqlalchemy import Column
from sqlalchemy import create_engine
from sqlalchemy import Integer, String
Base=declarative_base()
engine = create_engine('sqlite:///:memory:', echo=True)
class User(Base):
__tablename__ = 'user'
id = Column(Integer, primary_key=True)
name = Column(String(16))
tech_id = Column(Integer, unique=True, nullable=True)
Base.metadata.create_all(engine)
Session = sessionmaker(bind=engine)
s = Session()
Now we add two records:
u1=User(name="User1", tech_id=None)
u2=User(name="User2", tech_id=10)
s.add(u1)
s.add(u2)
s.commit()
Next try to modify them:
u1=s.query(User).filter(User.name=="User1").first()
u2=s.query(User).filter(User.name=="User2").first()
u2.tech_id=None
u1.tech_id=10
s.commit()
After commit I've got the exception:
<-- cut -->
sqlalchemy.exc.IntegrityError: (sqlite3.IntegrityError) UNIQUE constraint failed: user.tech_id [SQL: 'UPDATE user SET tech_id=? WHERE user.id = ?'] [parameters: ((10, 1), (None, 2))]
If I do like this:
u2.tech_id=None
s.commit()
u1.tech_id=10
s.commit()
It's all right.
Is it possible to do requests by only one commit (by only one transaction)?

Answer 1

You can do it in this way:
#u1=s.query(User).filter(User.name=="User1").first()
#u2=s.query(User).filter(User.name=="User2").first()
#u2.tech_id=None
#u1.tech_id=10
#s.commit()
s.query(User).filter(User.name=="User2").update(
{User.tech_id: None}
)
s.query(User).filter(User.name=="User1").update(
{User.tech_id: 10}
)
s.commit()
The essence of that changes is to perform right sequence of actions. You can't create duplicate values for unique key (at least in mysql) even before committing. But you can create multiple NULL values for unique column.