多对多第三张表的三种创建方式

• orm自动创建（可扩展性差，无法即时添加其他字段）

class Book(models.Model):
name = models.CharFiled(max_length=32)
authors = models.ManyToManyField(to="Author")

class Author(models.Model):
name = models.CharField(max_length=32)

• 手动创建第三张表（不支持直接跨表查询）

class Book(models.Model):
name = models.CharField(max_length=32)

class Author(models.Model):
name = models.CharFiled(max_length=32)

class Book2Author(models.Model):
book = models.ForeignKey(to="Book")
Author = models.ForeignKey(to="Author")
info = models.CharFiled(max_length=32)

手动创建第三张表，并通过字段进行关联（可扩展性搞，支持直接跨表查询）

class Book(models.Model):
name = models.CharFiled(max_length=32)
authors = models.ManyToManyField(to="Author", through="Book2Author", through_fields=("book", "author"))


class Author(models.Model):
name = models.CharFiled(max_length=32)
#books = models.Many2ManyField(to="Book", through="Book2Author", through_fields=("author", "book"))  # 与book表中的最后一个字段二选一

class Book2Author(models.Model):
book = models.ForeignKey(to="Book")
Author = models.ForeignKey(to="Author")
info = models.CharField(max_length=32)