如何使用SQLAlchemy 正确地向SQLite DB添加外键约束

参见英文答案 > Sqlite / SQLAlchemy: how to enforce Foreign Keys?                                    7个
我对SQLAlchemy很新,我想弄明白.

请记住以下测试设置：

class Nine(Base):
    __tablename__ = 'nine'
    __table_args__ = (sqlalchemy.sql.schema.UniqueConstraint('nine_b', name='uq_nine_b'), )

    nine_a = sqlalchemy.Column(sqlalchemy.dialects.sqlite.INTEGER(), primary_key=True, autoincrement=False, nullable=False)
    nine_b = sqlalchemy.Column(sqlalchemy.String(20), nullable=False)


class Seven(Base):
    __tablename__ = 'seven'
    __table_args__ = (sqlalchemy.sql.schema.PrimaryKeyConstraint('seven_a', 'seven_b'),
                      sqlalchemy.sql.schema.Index('fk_seven_c_nine_a_idx', 'seven_c'),)

    seven_a = sqlalchemy.Column(sqlalchemy.dialects.sqlite.INTEGER(), nullable=False)
    seven_b = sqlalchemy.Column(sqlalchemy.dialects.sqlite.INTEGER(), nullable=False)
    seven_c = sqlalchemy.Column(sqlalchemy.dialects.sqlite.INTEGER(), sqlalchemy.ForeignKey('nine.nine_a'), nullable=False)
    seven_d = sqlalchemy.Column(sqlalchemy.dialects.sqlite.INTEGER(), nullable=False)

    nine = sqlalchemy.orm.relationship(Nine, backref=sqlalchemy.orm.backref('seven'), uselist=False)


class Three(Base):
    __tablename__ = 'three'
    __table_args__ = (sqlalchemy.sql.schema.UniqueConstraint('three_b', 'three_c', name='uq_three_b_c'),
                      sqlalchemy.sql.schema.Index('fk_three_c_seven_a_idx', 'three_c'), )

    three_a = sqlalchemy.Column(sqlalchemy.dialects.sqlite.INTEGER(), primary_key=True, autoincrement=True, nullable=False)
    three_b = sqlalchemy.Column(sqlalchemy.dialects.sqlite.INTEGER(), nullable=False)
    three_c = sqlalchemy.Column(sqlalchemy.dialects.sqlite.INTEGER(), sqlalchemy.ForeignKey('seven.seven_a'), nullable=False)

    seven = sqlalchemy.orm.relationship(Seven, backref=sqlalchemy.orm.backref('three'), uselist=False)

这转化为以下DDL：

CREATE TABLE nine (
    nine_a INTEGER NOT NULL, 
    nine_b VARCHAR(20) NOT NULL, 
    PRIMARY KEY (nine_a), 
    CONSTRAINT uq_nine_b UNIQUE (nine_b)
);

CREATE TABLE seven (
    seven_a INTEGER NOT NULL, 
    seven_b INTEGER NOT NULL, 
    seven_c INTEGER NOT NULL, 
    seven_d INTEGER NOT NULL, 
    PRIMARY KEY (seven_a, seven_b), 
    FOREIGN KEY(seven_c) REFERENCES nine (nine_a)
);

CREATE INDEX fk_seven_c_nine_a_idx ON seven (seven_c);

CREATE TABLE three (
    three_a INTEGER NOT NULL, 
    three_b INTEGER NOT NULL, 
    three_c INTEGER NOT NULL, 
    PRIMARY KEY (three_a), 
    CONSTRAINT uq_three_b_c UNIQUE (three_b, three_c), 
    FOREIGN KEY(three_c) REFERENCES seven (seven_a)
);

CREATE INDEX fk_three_c_seven_a_idx ON three (three_c);

所有表都是空的.然后,以下代码语句：

session.add(Nine(nine_a=1, nine_b='something'))
session.add(Nine(nine_a=2, nine_b='something else'))
session.commit()

session.add(Seven(seven_a=7, seven_b=7, seven_c=7, seven_d=7))
session.commit()

session.add(Three(three_a=3, three_b=3, three_c=3))
sessionDB.commit()

有人可以解释为什么上面的代码片段没有错误执行？ FK约束不应该停止将新行插入七或三个？我假设在类本身中如何描述FK有什么问题,但我不知道问题出在哪里(以及如何修复它).

[编辑1]

为所有类添加__table_args__(忘记包含它们).

[编辑2]

添加DDL以供进一步参考.

解决方法:

SQLite默认情况下不强制执行ForeignKey约束(参见此处http://www.sqlite.org/pragma.html#pragma_foreign_keys)

要启用,请按以下文档操作：http://docs.sqlalchemy.org/en/latest/dialects/sqlite.html#foreign-key-support

这是官方文档的复制粘贴：

SQLite supports FOREIGN KEY syntax when emitting CREATE statements for tables, however by default these constraints have no effect on the operation of the table.

Constraint checking on SQLite has three prerequisites:

• At least version 3.6.19 of SQLite must be in use
• The SQLite library must be compiled without the SQLITE_OMIT_FOREIGN_KEY or SQLITE_OMIT_TRIGGER symbols enabled.
• The PRAGMA foreign_keys = ON statement must be emitted on all connections before use.
  SQLAlchemy allows for the PRAGMA statement to be emitted automatically for new connections through the usage of events:

from sqlalchemy.engine import Engine
from sqlalchemy import event

@event.listens_for(Engine, "connect")
def set_sqlite_pragma(dbapi_connection, connection_record):
    cursor = dbapi_connection.cursor()
    cursor.execute("PRAGMA foreign_keys=ON")
    cursor.close()

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