Detecting breach of unique constraint in SQLAlchemy

Question

Because Country.name in the following code sample has unique=True , and because two countries of the same name are created, I'm expecting that session.commit() would fail.

from sqlalchemy import Column, Integer, String
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker


Base = declarative_base()


class Country(Base):
    __tablename__ = 'countries'
    id = Column(Integer, primary_key=True, autoincrement=True)
    name = Column(String(64), unique=True, nullable=False, index=True)


engine = create_engine('sqlite:///countries_example.db')
Base.metadata.create_all(engine)
Base.metadata.bind = engine
DBSession = sessionmaker(bind=engine)
session = DBSession()

malta1 = Country(name='Malta')
malta2 = Country(name='Malta')
session.add(malta1)
session.add(malta2)
session.commit()

But the commit proceeds just fine. How do I detect that the unique constraint was breached?

Answer 1

When I run your code as written, it does fail:

Traceback (most recent call last):
  File "so4.py", line 26, in <module>
    session.commit()
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/sqlalchemy/orm/session.py", line 906, in commit
    self.transaction.commit()
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/sqlalchemy/orm/session.py", line 461, in commit
    self._prepare_impl()
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/sqlalchemy/orm/session.py", line 441, in _prepare_impl
    self.session.flush()
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/sqlalchemy/orm/session.py", line 2177, in flush
    self._flush(objects)
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/sqlalchemy/orm/session.py", line 2297, in _flush
    transaction.rollback(_capture_exception=True)
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/sqlalchemy/util/langhelpers.py", line 66, in __exit__
    compat.reraise(exc_type, exc_value, exc_tb)
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/sqlalchemy/util/compat.py", line 187, in reraise
    raise value
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/sqlalchemy/orm/session.py", line 2261, in _flush
    flush_context.execute()
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/sqlalchemy/orm/unitofwork.py", line 389, in execute
    rec.execute(self)
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/sqlalchemy/orm/unitofwork.py", line 548, in execute
    uow
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/sqlalchemy/orm/persistence.py", line 181, in save_obj
    mapper, table, insert)
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/sqlalchemy/orm/persistence.py", line 835, in _emit_insert_statements
    execute(statement, params)
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/sqlalchemy/engine/base.py", line 945, in execute
    return meth(self, multiparams, params)
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/sqlalchemy/sql/elements.py", line 263, in _execute_on_connection
    return connection._execute_clauseelement(self, multiparams, params)
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/sqlalchemy/engine/base.py", line 1053, in _execute_clauseelement
    compiled_sql, distilled_params
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/sqlalchemy/engine/base.py", line 1189, in _execute_context
    context)
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/sqlalchemy/engine/base.py", line 1402, in _handle_dbapi_exception
    exc_info
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/sqlalchemy/util/compat.py", line 203, in raise_from_cause
    reraise(type(exception), exception, tb=exc_tb, cause=cause)
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/sqlalchemy/util/compat.py", line 186, in reraise
    raise value.with_traceback(tb)
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/sqlalchemy/engine/base.py", line 1182, in _execute_context
    context)
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/sqlalchemy/engine/default.py", line 470, in do_execute
    cursor.execute(statement, parameters)
sqlalchemy.exc.IntegrityError: (sqlite3.IntegrityError) UNIQUE constraint failed: countries.name [SQL: 'INSERT INTO countries (name) VALUES (?)'] [parameters: ('Malta',)]

The unique constraint is part of the table, not necessarily the model. When I run the database creation first without it, then run your code, I get no problems.

For a more complicated table, you would want to use a migration tool like alembic, but for this, you could go ahead and do it yourself (you can do it in one script, but I just do two to keep it simple, sorry if that's not pythonic:

from sqlalchemy import Column, Integer, String
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker


Base = declarative_base()


class Country(Base):
    __tablename__ = 'countries'
    id = Column(Integer, primary_key=True, autoincrement=True)
    name = Column(String(64), unique=True, nullable=False, index=True)

class TempCountry(Base):
    __tablename__ = 'temp_countries'
    id = Column(Integer, primary_key=True, autoincrement=True)
    name = Column(String(64), nullable=False, index=True)

engine = create_engine('sqlite:///countries_example.db')
Base.metadata.create_all(engine)
Base.metadata.bind = engine
DBSession = sessionmaker(bind=engine)
session = DBSession()

foreach country in session.query(Country).all():
    if len(session.query(TempCountry).filter_by(name=country.name).all()) == 0:
        temp_country = TempCountry(id=country.id, name=country.name)
        session.add(temp_country)
        session.commit()

Country.__table__.drop(engine)
session.commit()

followed by

from sqlalchemy import Column, Integer, String
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker


Base = declarative_base()


class Country(Base):
    __tablename__ = 'countries'
    id = Column(Integer, primary_key=True, autoincrement=True)
    name = Column(String(64), unique=True, nullable=False, index=True)

class TempCountry(Base):
    __tablename__ = 'temp_countries'
    id = Column(Integer, primary_key=True, autoincrement=True)
    name = Column(String(64), nullable=False, index=True)

engine = create_engine('sqlite:///countries_example.db')
Base.metadata.create_all(engine)
Base.metadata.bind = engine
DBSession = sessionmaker(bind=engine)
session = DBSession()

foreach temp_country in session.query(TempCountry).all():
    country = Country(id=country.id, name=country.name)
    session.add(country)

session.commit()

TempCountry.__table__.drop(engine)
session.commit()

You don't have to pass the id, but you can if you need to keep it for whatever reason. Also, remember to commit before dropping the table or it won't work quite right.