How to join 3 tables in query with Django

Question

models.py

class Employee(models.Model):
    emp_no = models.IntegerField(primary_key=True)
    first_name = ...
    last_name = ...

    # emp_no first_name  last_name
      ------ ----------  ----------
      10005  Christian   Erde

class DeptEmp(models.Model):
    emp_no = models.ForeignKey(Employee, on_delete=models.CASCADE)
    dept_no = models.ForeignKey(Department, on_delete=models.CASCADE)

    # dept_no_id  emp_no_id 
      ----------  ----------
      d003        10005     

class Department(models.Model):
    dept_no = models.CharField(primary_key=True, max_length=4)
    dept_name = models.CharField(unique=True, max_length=40)

    #  dept_no     dept_name      
       ----------  ---------------
       d003        Human Resources

views.py

class EmpList(ListView):
    queryset = Employee.objects.all().select_related('deptemp').select_related('department')

employee_list.html

{% for emp in object_list %}
    {{ emp.first_name }}
    {{ emp.last_name }}
    {{ emp.department.dept_name }}
{% endfor %}

I got this error message on the browser: Invalid field name(s) given in select_related: 'deptemp', 'department'. Choices are: (none)

Solution

"select_related"

Returns a QuerySet that will “follow” foreign-key relationships.

from django.db import models
class City(models.Model):
    # ...
    pass
class Person(models.Model):
    # ...
    hometown = models.ForeignKey(
        City,
        on_delete=models.SET_NULL,
        blank=True,
    null=True,
    )

class Book(models.Model):
    # ...
    author = models.ForeignKey(Person, on_delete=models.CASCADE)

then a call to Book.objects.select_related('author__hometown')

For your exmaples:

class EmpList(ListView):
    queryset = DeptEmp.objects.all().select_related('emp_no').select_related('dept_no')

employee_list.html

{% for deptemp in object_list %}
    {{ deptemp.emp_no.first_name }}
    {{ deptemp.emp_no.last_name }}
    {{ deptemp.dept_no.dept_name }}
{% endfor %}