多对多表关系三种建立方式

    第一种:ManyToMany自动创建(需要掌握)
    第二种:纯手动创建(了解即可)
        class Book(models,Model):
            name = ...
        class Author(models,Model):
            name = ...
        class Book2Author(models,Model):
            book = ...
            author=...
    第三种:半自动创建(需要掌握)

        class Book(models,Model):
            name = ...
            authors = (through,through_fields=('book','author'))
        class Author(models,Model):
            name = ...
            
        class Book2Author(models,Model):
            book = ...
            author=...