SQLAlchemy确定是否存在唯一约束

Question

我有一个要运行验证的SQLAlchemy模型。验证的一部分是确保（少数）列上存在UniqueConstraint。我知道列是什么。使用SQLAlchemy可以做到这一点吗？我正在使用的基础数据库是MySQL。

Answer 1

您可以使用SQLalchemy反射API。

为了获得唯一约束，请发出get_unique_constraints。

主键是唯一的，因此您也必须发出get_pk_constraint。

创建的表：

CREATE TABLE user (
    id INTEGER NOT NULL, 
    name VARCHAR(255), 
    email VARCHAR(255), 
    login VARCHAR(255), 
    PRIMARY KEY (id), 
    UNIQUE (email), 
    UNIQUE (login)
)

例：

from sqlalchemy import create_engine
from sqlalchemy.engine.reflection import Inspector

# engine = create_engine(...)

insp = Inspector.from_engine(engine)

print "PK: %r" % insp.get_pk_constraint("user")
print "UNIQUE: %r" % insp.get_unique_constraints("user")

输出：

PK: {'name': None, 'constrained_columns': [u'login']}
UNIQUE: [{'column_names': [u'email'], 'name': None}, {'column_names': [u'login'], 'name': None}]

您可以通过以下方式验证唯一约束：

pk = insp.get_pk_constraint("user")['constrained_columns']
unique = map(lambda x: x['column_names'], insp.get_unique_constraints("user"))

for column in ['name', 'id', 'email', 'login']:
    print "Column %r has an unique constraint: %s" %(column, [column] in [pk]+unique)

输出：

Column 'name' has an unique constraint: False
Column 'id' has an unique constraint: True
Column 'email' has an unique constraint: True
Column 'login' has an unique constraint: True

更新01

如果您想在创建表之前更简单地检查列，则上面的代码仅检查已创建表的列约束：

from sqlalchemy import create_engine, Column, types
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker, scoped_session

Base = declarative_base()

class User(Base):
    __tablename__ = "user"
    id = Column(types.Integer, primary_key=True)
    name = Column(types.String(255))
    email = Column(types.String(255), unique=True)
    login = Column(types.String(255), unique=True)

# do not create any table
#engine = create_engine('sqlite:///:memory:', echo=True)
#session = scoped_session(sessionmaker(bind=engine))
#Base.metadata.create_all(engine)

# check if column is (any) a primary_key or has unique constraint
# Note1: You can use User.__table__.c too, it is a alias to columns
# Note2: If you don't want to use __table__, you could use the reflection API like:
#        >>> from sqlalchemy.inspection import inspect
#        >>> columns = inspect(User).columns
result = dict([(c.name, any([c.primary_key, c.unique])) for c in User.__table__.columns])

print(result)

输出：

{'email': True, 'login': True, 'id': True, 'name': False}

如果只想检查一些列，则只能执行以下操作：

for column_name in ['name', 'id', 'email', 'login']:
    c = User.__table__.columns.get(column_name)
    print("Column %r has an unique constraint: %s" %(column_name, any([c.primary_key, c.unique])))

输出：

Column 'name' has an unique constraint: False
Column 'id' has an unique constraint: True
Column 'email' has an unique constraint: True
Column 'login' has an unique constraint: True