the model layer

    Model：A model is the single, definitive source of information about your data. It contains the essential fields and behaviors of the data you’re storing. Generally, each model maps to a single database table.

    model 的创建。

    ① ob = Object(); ob.attr='*'; ob.save();

    ② ob = Object(attr='*'); ob.save();

    ③ ob = Object.objects.create(attr='*'); ob.save();

    model的对象中的多对多，多对一，一对一关系。

        多对多：models.ManyToManyField(Model_name)

        多对一：models.ForeignKey(Model_name)