Question

如何在Django（ORM）中联接3个或更多表并获取结果？
已创建3个模型：

1.student 2.标记 3.细节

class Student:    
    s_id = models.IntegerField(primary_key=True)    
    s_name = models.CharField(max_length=100)  

class Marks:  
    school_id = models.IntegerField(primary_key=True)  
    s_id = models.ForeignKey(Student, on_delete=models.CASCADE)  
    score = models.IntegerField()  
    status = models.CharField(max_length=30)  

class Details:  
    address_city = models.CharField(max_length=30)  
    emailid = models.EmailField()  
    school_id = models.ForeignKey(Marks,on_delete=models.CASCADE)  
    accomplishments = models.TextField()

我需要加入这3个表并获取学生的姓名，得分，状态，地址城市，电子邮件ID，成就。
在SQL中，我们可以这样写：

select s_name, score, status, address_city, email_id,
accomplishments from student s inner join marks m on
s.s_id = m.s_id inner join details d on 
d.school_id = m.school_id;

请让我知道如何使用DJANGO代码实现相同的目的。

Answer 1

对给定型号的一些建议：

models.py：

class Student:    
    name = models.CharField(max_length=100)  

class Marks:  
    student = models.ForeignKey(Student, on_delete=models.CASCADE)  
    score = models.IntegerField()  
    status = models.CharField(max_length=30)  

class Details:  
    address_city = models.CharField(max_length=30)  
    emailid = models.EmailField()  
    school = models.ForeignKey(Marks,on_delete=models.CASCADE)  
    accomplishments = models.TextField()

现在，当我们在sql中创建映射表时，创建映射模型：

class StundetMarksDetailsMap:
  student = models.ForeignKey(Student, on_delete=models.CASCADE)
  marks = models.ForeignKey(Marks, on_delete=models.CASCADE)
  details =  models.ForeignKey(Details, on_delete=models.CASCADE)

现在，您可以根据需要在views.py或所需位置创建自己的queryset：

class StudentView(generic.DetailsView):
  model = Student
  template = student_details.html

  //this will return all values related student
    def get_queryset(self):
        return StundetMarksDetailsMap.objects.filter(student=Student)

基本上，此get_queryset返回与所选择的stundet相关的模型对象，使用返回的Details和Marks对象在模板中调用它们的相应字段。还请仅使用要显示的字段。

希望这对您也有用。最好的：）

Answer 2

我认为您应该使它保持简单，而不是使模型复杂化，当您拥有大量数据时，这种模型就不好了。将学生模型作为主要班级。并将所有其他信息存储在其他表中。

class Student:    
    name = models.CharField(max_length=100)  
    marks = models.ForeignKey(Marks,on_delete=models.CASCADE)  
    school = models.ForeignKey(School,on_delete=models.CASCADE)

class Marks:    
    score = models.IntegerField()  
    status = models.CharField(max_length=30)  

class Details:  
    address_city = models.CharField(max_length=30)  
    emailid = models.EmailField()  
    accomplishments = models.TextField() 

class School:
     name = models.CharField(max_length=30)
     details = models.ForeignKey(Details)

现在执行以下查询

student = Student.objects.filter(pk=student_id).filter(school_pk=school_id)

print student.name
print student.marks.score
print student.marks.status
print student.details.address_city

对于您当前的模型，很难查询。但是您可以尝试编写SQL查询本身。像这样

Blog.objects.extra(
    select={
        'entry_count': 'SELECT COUNT(*) FROM blog_entry WHERE blog_entry.blog_id = blog_blog.id'
    },
)

关注此 https://docs.djangoproject.com/en/2.1/ref/models/querysets/#extra

Django模型-DJANGO中的SQL等效多个表联接

2 个答案: