在SQLAlchemy中检测违反唯一约束的情况

Question

由于以下代码示例中的Country.name具有unique=True ，并且由于创建了两个具有相同名称的国家，所以我期望session.commit()会失败。

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()

但是提交进行得很好。 如何检测到唯一约束已被违反？

Answer 1

当我以书面形式运行您的代码时，它的确失败了：

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',)]

唯一约束是表的一部分，不一定是模型。 当我首先运行没有它的数据库创建，然后运行您的代码时，我没有任何问题。

对于更复杂的表，您可能想要使用像alembic这样的迁移工具，但是为此，您可以自己进行操作（您可以在一个脚本中进行操作，但是我只做两个来简化操作，抱歉那不是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()

其次是

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()

您不必传递ID，但是如果出于任何原因需要保留ID，则可以传递。 另外，请记住在删除表之前先进行提交，否则它将无法正常工作。