1、一对一查询

    models.OneToOneField(Entry)
    两个实体类：Author , Wife
        1、通过 Author 找 Wife
            author = Author.objects.get(id=1)
            wife = author.wife
        2、通过 Wife 找 Author
            wife = Wife.objects.get(id=2)
            author = wife.author

2、一对多/多对一查询

    model = models.ForeignKey(Entry)
    两个实体类：Publisher(一) , Book(多)
    正向查询：
        book = Book.objects.get(id=1)
        publisher = book.publisher
    反向查询
        publisher = Publisher.objects.get(id=4)
        books = publisher.book_set.all()

3、多对多查询

    1、什么是多对多
        A表中的一条记录可以与B表中的任意多条记录匹配，同时B表中的每一条记录也可以与A表中的任意多条记录相匹配
    2、语法
        entry = models.ManyToManyField(Entry)
    3、查询
        class Author(models.Model):
            ... ...
            publish = models.ManyToManyField(Publish)
        
        正向查询：在 Author 中查询 Publish
            author = Author.objects.get(id=3)
            pub_list=author.publish.all()

            通过关联属性.all()
        反向查询：在 Publisher 中查询 Author
            pub = Publisher.objects.get(id=1)
            auList = pub.author_set.all()