外键需要键的表中的值来匹配另一个表中的列

请原谅,代码太多了,但我不确定是否可以用其他方式解释我的问题

class Project(models.Model):
    name = models.CharField(max_length=100, unique=True)
    dir  = models.CharField(max_length=300, blank=True, unique=True )

    def __unicode__(self):
        return self.name;

class ASClass(models.Model):
    name      = models.CharField(max_length=100)
    project   = models.ForeignKey(Project, default=1)

    def __unicode__(self):
        return self.name;

class Entry(models.Model):
    project     = models.ForeignKey(Project, default=1)
    asclasses   = models.ManyToManyField(ASClass)

是否有一种方法,在不重写模型的save函数的情况下,使条目只允许具有相同项目ID的类?

明确地说,我并不反对覆盖save。事实上,我已经在本案中否决了它,以提供上面未列出的财产。我已经知道如何通过简单地扩展覆盖来回答这个问题,所以简单地说“您可以覆盖保存”是没有帮助的。

我想知道是否有更好的方法来实现这一点,是否有Django本机实现,以及密钥类型是否已经存在。

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在Postgresql中也有这样做的方法吗?

(为了更好地衡量,下面是在Postgresql中创建表的代码) 这创建了以下表格:

CREATE TABLE blog_asclass
(
  id serial NOT NULL,
  "name" character varying(100) NOT NULL,
  project_id integer NOT NULL,
  CONSTRAINT blog_asclass_pkey PRIMARY KEY (id),
  CONSTRAINT blog_asclass_project_id_fkey FOREIGN KEY (project_id)
      REFERENCES blog_project (id) MATCH SIMPLE
      ON UPDATE NO ACTION ON DELETE NO ACTION DEFERRABLE INITIALLY DEFERRED
)

CREATE TABLE blog_entry
(
  id serial NOT NULL,
  project_id integer NOT NULL,
  build_date timestamp with time zone NOT NULL,
  CONSTRAINT blog_entry_pkey PRIMARY KEY (id),
  CONSTRAINT blog_entry_project_id_fkey FOREIGN KEY (project_id)
      REFERENCES blog_project (id) MATCH SIMPLE
      ON UPDATE NO ACTION ON DELETE NO ACTION DEFERRABLE INITIALLY DEFERRED
)

CREATE TABLE blog_entry_asclasses
(
  id serial NOT NULL,
  entry_id integer NOT NULL,
  asclass_id integer NOT NULL,
  CONSTRAINT blog_entry_asclasses_pkey PRIMARY KEY (id),
  CONSTRAINT blog_entry_asclasses_asclass_id_fkey FOREIGN KEY (asclass_id)
      REFERENCES blog_asclass (id) MATCH SIMPLE
      ON UPDATE NO ACTION ON DELETE NO ACTION DEFERRABLE INITIALLY DEFERRED,
  CONSTRAINT blog_entry_asclasses_entry_id_fkey FOREIGN KEY (entry_id)
      REFERENCES blog_entry (id) MATCH SIMPLE
      ON UPDATE NO ACTION ON DELETE NO ACTION DEFERRABLE INITIALLY DEFERRED,
  CONSTRAINT blog_entry_asclasses_entry_id_key UNIQUE (entry_id, asclass_id)
)

CREATE TABLE blog_project
(
  id serial NOT NULL,
  "name" character varying(100) NOT NULL,
  dir character varying(300) NOT NULL,
  CONSTRAINT blog_project_pkey PRIMARY KEY (id),
  CONSTRAINT blog_project_dir_key UNIQUE (dir),
  CONSTRAINT blog_project_name_key UNIQUE (name)
)