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Doing things with Django models - aka - Django models tutorial

By : Shabda Raaj

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.

An 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.)

So Employee has a field department = models.ForeignKey("Department"). See that the ForeignKey field was added on the class which has in many. In database, this creates a FK from in Employee table which refernces Departments.

A 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.

A 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.

Accessing objects.

Getting a specific employee.
Employee.objects.get(pk = someval)
Employee.objects.get(name= someval)
Given an department get all employees.
department.employee_set.all()

The department gets an attribute name <FKClass>_set.

Given an employee, get all colleagues.

employee.department.employee_set.objects.all()

To get siblings, get parents, and get all children.

Given an department, get number of employees.

department.employee_set.all().count()

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,

As one Department will have one Employee as hod, and Employee can head at max one Department, we have a one to one relationship
As many Employee will report to another Employee, so we have a FK on Employee, referencing self. As Employee will take many EmployeeLeave, EmployeeLeave has a FK to Epployee

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.

contractor.department_set.all()

Get a list of all contractor who work for a department.

department.contractor_set.all()

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(__gte). Also we specified two filter conditions, so they were ANDed together.

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.


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Comments

Kenneth

"List of all managers in a given department."

Assuming managers don't have managers themselves:
department.employee_set.objects.filter(manager=None)

commmenttor
shabda

@Kenneth, well its a large company with hierarchical management, so managers have managers. (WIth pointy hairs).

commmenttor
valKirian 19th Jan., 2010


List of all managers in a given department.

dept = Department.objects.all()[0]
Employee.objects.filter(department=dept, manager__isnull=True)

commmenttor
shabda

@valKirian

Same as above,

you are assuming that managers have no managers. :)

commmenttor
erwt 19th Jan., 2010

CAn you make a screenshot of the tables and the relations?
It helps to understand the tutorial

Thanks

commmenttor
Franco 19th Jan., 2010

“List of all managers in a given department.”

managers = Employee.objects.filter(manager__isnull=False).filter(department__name = "mydepartment")

commmenttor
shabda

@ Franco: Read as above. :)

commmenttor
Matt 21st Jan., 2010

“List of all managers in a given department.”

Haven't tested it, but something like this:
managers=Employee.objects.filter(employee__isnull=False,department=dept)

manager is an employee to employee relationship so the reverse relationship (based on the django docs) will be available as employee_set as an attribute and in query terms to traverse it you'd use employee.

It'd be made clearer by defining the manager field on Employee as:
manager = models.ForeignKey("self", blank = True, null = True, related_name="manages")

So you could then have:
managers=Employee.objects.filter(manages__isnull=False,department=dept)

commmenttor
matt 8th July, 2010

great post, helped clear up some concepts for me.

commmenttor
test

I've truly said that will least 4173215 occasions. SCK seemed to be here

commmenttor

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Doing things with Django models – aka – Django models tutorial: Django abstracts most of the actions you would be … http://bit.ly/8irAKE

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http://uswaretech.com/blog/2010/01/django-models-tutorial/ Doing things with Django models aka Django models tutorial.

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Doing things with Django models – aka – Django models tutorial http://goo.gl/fb/ZM9B #django

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Doing things with Django models – aka – Django models tutorial — The Usware Blog – Django Web Development: http://bit.ly/5LiCOz

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