Django abstracts most of the actions you would be doing with the Database. What it doesn’t abstracts, and doesn’t try to abstract is the Database modelling part. This is a quick tutorial describing to how model your data in Django models.py, and how to access and modify them.
Consider a hypothetical HR department, which wants you to build an application to track and manage their processes. They have employees who work for a department, contractors who work for multiple department. Let’s see how you you would do that in Django.
from django.db import models class Employee(models.Model): name = models.CharField(max_length = 100) department = models.ForeignKey("Department") class Department(models.Model): name = models.CharField(max_length = 100) class EmployeeHistory(models.Model): employee = models.OneToOneField(Employee) date_joined = models.DateField() marital_status = models.BooleanField() class Contactor(models.Model): name = models.CharField(max_length = 100) departments = models.ManyToManyField(Department)
Let’s see the type of relationship we created here.
Employee has a Many-to-one relationship with
Department, (i.e. One department will have many
Employee, but one employee will have a single departments.)
Employee has a field
department = models.ForeignKey("Department"). See that the
was added on the class which has in many. In database, this creates a FK from in
Employee table which refernces
Contractor has many-to-many relationships with
Department, so it has a
ManyToMany field. This field can be created
on either classes. At a database level this creates a new table which has a FK to both the tables.
Employee has one-to-one relationship with
EmployeeHistory. The thing to note here is that whatever we could do with
OneToOneField, we can do by including the fields in the Other Class directly. However when there are fields which are only rarely
needed with a given model, it is useful to separate them via a one-to-one field. The one to one field can be on either class.
Getting a specific employee.
Employee.objects.get(pk = someval) Employee.objects.get(name= someval)
Given an department get all employees.
department gets an attribute name
Given an employee, get all colleagues.
To get siblings, get parents, and get all children.
Given an department, get number of employees.
Ok so the HR department bosses are happy with what they see, and ask you to track this data, who is the manager of each employee, who is the HOD of each department and when did each employee took leave.
from django.db import models class Employee(models.Model): name = models.CharField(max_length = 100) manager = models.ForeignKey("self", blank = True, null = True) department = models.ForeignKey("Department") class Department(models.Model): hod = models.ForeignKey("Employee") name = models.CharField(max_length = 100) class EmployeeHistory(models.Model): employee = models.OneToOneField(Employee) date_joined = models.DateField() marital_status = models.BooleanField() class Contactors(models.Model): name = models.CharField(max_length = 100) departments = models.ManyToManyField(Department) class EmployeeLeave(models.Model): leave_taken = models.DateField() employee = models.ForeignKey(Employee)
So we now have new fields
hod = models.OneToOneField("Employee") in department and
manager = models.ForeignKey("self", blank = True, null = True) and a new model
EmployeeLeave. Let su see
the new realtions,
Department will have one
Employee as hod, and
Employee can head at max one
Department, we have a one to one relationship
Employee will report to another
Employee, so we have a FK on
Employee will take many
EmployeeLeave has a FK to
Let us see some queries.
Get all leaves taken by an employee this year
import datetime today = datetime.date.today() start_of_year = datetime.datetime(today.year, 1, 1) leaves_taken = employee.employeeleave_set.filter(leave_taken__gt=start_of_year, leave_taken__lt = today)
Get a list of all HODs.
#As there are going to be as many Employees as departments departments = Department.objects.all() employees = [department.hod for department in departments]
Get a list of all departments a contractor works for.
Get a list of all contractor who work for a department.
Note that each side of a many to many relationship get a Manager.
List of all managers in a given department.
Todo#(Can't think of any one query way to do this. If you know, let me know :) )
List of all leaves taken in a given department.
EmployeeLeave.objects.filter(employee__department = department)
None that we used double underscores
__ to do a filtering on a field across Entities.
List of all employees which joined a given department this year.
Employee.objects.filter(department = department, employment_history__date_joined__gte=start_of_year, )
Note that we used a double underscore
__ twice, first to go across entities, and then to define the type of filter(
Also we specified two filter conditions, so they were
List all employees which either report too a given employee, or joined before him.
Employee.objects.filter(Q(manager = employee)|Q(employee_history__date_joined__lt = employee.employee_history.date_joined))
Note the new construct
Q, they are used to specify complex boolean operations. Here we used the
| (or operator) to specify or condition.
Thank you for reading the Agiliq blog. This article was writen by shabda on Jan 18, 2010 in models .
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