Peewee 循环外键依赖异常

Question

我正在尝试使用循环依赖复制 Peewee 示例应用程序，如 the docs 中所述，尽管创建者明确表示这通常不是一个好主意。这是主要从文档中复制的代码：

from peewee import *

db = SqliteDatabase(None)

class BaseModel(Model):

class Meta:
    database = db

class User(BaseModel):
    username = CharField()
    # Tweet has not been defined yet so use the deferred reference.
    favorite_tweet = DeferredForeignKey('Tweet', null=True)

class Tweet(BaseModel):
    message = TextField()
    user = ForeignKeyField(User, backref='tweets')


db.init('twitter.db')
db.create_tables([User, Tweet])
User._schema.create_foreign_key(User.favorite_tweet) #Error
db.close()

我在用#Error 注释的行中遇到异常。需要这一行，如文档中所述：

When you call create_table we will again encounter the same issue. For this reason peewee will not automatically create a foreign key constraint for any deferred foreign keys.

To create the tables and the foreign-key constraint, you can use the SchemaManager.create_foreign_key() method to create the constraint after creating the tables.

这是我使用 Python 3.5.2:

时遇到的异常

Traceback (most recent call last):
  File "/usr/local/lib/python3.5/dist-packages/peewee.py", line 2653, in execute_sql
    cursor.execute(sql, params or ())
sqlite3.OperationalError: near "CONSTRAINT": syntax error

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "test3.py", line 23, in <module>
    User._schema.create_foreign_key(User.favorite_tweet)
  File "/usr/local/lib/python3.5/dist-packages/peewee.py", line 4930, in create_foreign_key
    self.database.execute(self._create_foreign_key(field))
  File "/usr/local/lib/python3.5/dist-packages/peewee.py", line 2666, in execute
    return self.execute_sql(sql, params, commit=commit)
  File "/usr/local/lib/python3.5/dist-packages/peewee.py", line 2660, in execute_sql
    self.commit()
  File "/usr/local/lib/python3.5/dist-packages/peewee.py", line 2451, in __exit__    reraise(new_type, new_type(*exc_args), traceback)
  File "/usr/local/lib/python3.5/dist-packages/peewee.py", line 178, in reraise
    raise value.with_traceback(tb)
  File "/usr/local/lib/python3.5/dist-packages/peewee.py", line 2653, in execute_sql
    cursor.execute(sql, params or ())
peewee.OperationalError: near "CONSTRAINT": syntax error

Answer 1

Sqlite 不支持 ALTER TABLE ADD CONSTRAINT——所以当您使用 Sqlite 时，您应该省略对 create_foreign_key() 的额外调用。

文档中有一个明确的说明：

Because SQLite has limited support for altering tables, foreign-key constraints cannot be added to a table after it has been created.

Peewee 循环外键依赖异常

Peewee circular foreign key dependencies exception

python

peewee