为什么我不能在编辑后保存我的模型实例？

Question

我有一个模型,我可以很好地实例化,但一旦创建,如果我试图保存它我得到一个IntegrityError说主键必须是唯一的.是什么导致了这个？

还有其他模型继承自Node,他们给了我同样的问题.

这是我的模特:

class Node(MPTTModel):
    title = models.CharField(max_length=255)
    parent = models.ForeignKey('self', null=True, blank=True,
                               related_name='children')

class Book(Node):
    isbn13 = models.CharField(max_length=14)
    description = models.TextField()

这是它生成的SQL:

CREATE TABLE "main_node" (
    "id" integer NOT NULL PRIMARY KEY,
    "title" varchar(255) NOT NULL,
    "parent_id" integer,
    "type" varchar(1) NOT NULL,
    "lft" integer unsigned NOT NULL,
    "rght" integer unsigned NOT NULL,
    "tree_id" integer unsigned NOT NULL,
    "level" integer unsigned NOT NULL
)
;
CREATE TABLE "main_book" (
    "node_ptr_id" integer NOT NULL PRIMARY KEY REFERENCES "main_node" ("id"),
    "isbn13" varchar(14) NOT NULL,
    "description" text NOT NULL
)
;

当我尝试保存预先存在的模型实例时会发生什么:

>>> b = Book.objects.create(title='c+ for dummies', isbn13='111-1111111111', description='nt')
>>> b.title='c++ for dummies'
>>> b.save()
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "C:\Users\Jackie\Desktop\Code\ProjectName\main\models.py", line 56, in save
    super(Book, self).save(self, *args)
  File "C:\Python27\lib\site-packages\django\db\models\base.py", line 460, in save
    self.save_base(using=using, force_insert=force_insert, force_update=force_update)
  File "C:\Python27\lib\site-packages\django\db\models\base.py", line 553, in save_base
    result = manager._insert(values, return_id=update_pk, using=using)
  File "C:\Python27\lib\site-packages\django\db\models\manager.py", line 195, in _insert
    return insert_query(self.model, values, **kwargs)
  File "C:\Python27\lib\site-packages\django\db\models\query.py", line 1436, in insert_query
    return query.get_compiler(using=using).execute_sql(return_id)
  File "C:\Python27\lib\site-packages\django\db\models\sql\compiler.py", line 791, in execute_sql
    cursor = super(SQLInsertCompiler, self).execute_sql(None)
  File "C:\Python27\lib\site-packages\django\db\models\sql\compiler.py", line 735, in execute_sql
    cursor.execute(sql, params)
  File "C:\Python27\lib\site-packages\django\db\backends\util.py", line 34, in execute
    return self.cursor.execute(sql, params)
  File "C:\Python27\lib\site-packages\django\db\backends\sqlite3\base.py", line 234, in execute
    return Database.Cursor.execute(self, query, params)
IntegrityError: PRIMARY KEY must be unique

以下是运行的查询:

>>> for q in connection.queries:
...   print q['sql'] + "\n\n"
...
SELECT MAX("main_node"."tree_id") AS "tree_id__max" FROM "main_node"


INSERT INTO "main_node" ("title", "parent_id", "type", "lft", "rght", "tree_id", "level") VALUES (c+
 for dummies, None, B, 1, 2, 1, 0)


INSERT INTO "main_book" ("node_ptr_id", "isbn13", "description") VALUES (1, 111-1111111111, nt)




SELECT (1) AS "a" FROM "main_node" WHERE "main_node"."id" = 1  LIMIT 1


UPDATE "main_node" SET "title" = c++ for dummies, "parent_id" = NULL, "type" = B, "lft" = 1, "rght"
= 2, "tree_id" = 1, "level" = 0 WHERE "main_node"."id" = 1


INSERT INTO "main_book" ("node_ptr_id", "isbn13", "description") VALUES (1, 111-1111111111, nt)

当我重新保存已创建的实例时,为什么要尝试插入main_book？

Answer 1

您必须对数据库有一些您不知道的唯一约束，或者您的数据库表的自动增量计数器已重置为某个无效值。

此链接似乎很有价值： http://old.nabble.com/Reset-auto-increment---truncate-td16494119.html

像这样考虑一下，自动增量计数器正在为已经存在的主键选择值，因为计数器的值设置为低于表中最后一个条目的值，因此当它选择下一个值时，它会选择一个值那已经存在了。

更新：发生了一些奇怪的事情。从您问题中的 SQL 查询来看，它似乎再次尝试 INSERT。尝试：

b.save(force_update=True)

这种特殊的场景似乎并不能保证force_update=True，但可能是 MPTT 或 Django 代码中的错误。