Episode 7 - Models and Managers and Querysets, Oh My!

On this episode, we will explore more about models and how to interact with data in your database.

Listen at Spotify.

Last Episode

On the last episode, we discussed the basics of setting up a database and creating a model to store data.

Working With Models

To create new rows in our new database tables, we can use a model’s save method. When you save a model instance, Django will send a message to the database that effectively says “add this new data to this database table.” These database “messages” are actually called queries.

What does an SQL query look like? If we save our model example from the last episode, it would look something like:

INSERT INTO "application_employee" ("first_name", "last_name", "job_title")
    VALUES ('Tom', 'Bombadil', 'Old Forest keeper')

ORM stands for Object Relational Mapper. The job of an ORM is to map (or translate) from Python objects to a relational database. This means that we spend our time working in Python code, and we let Django figure out how to get and put data into the database.

Using save on a model record is such a small example of the Django ORM. What else can we do? We can do things like:

  • Get all rows from the database.
  • Get a filtered set of rows based on some filtering criteria.
  • Update a set of rows at the same time.
  • Delete rows from the database.

We can analyze our fictitious employee table. The manager for a model is attached to the model class as an attribute named objects. Let’s see some code:

>>> from application.models import Employee
>>> bobs = Employee.objects.filter(first_name='Bob')
>>> for bob in bobs:
...     print(f"{bob.first_name} {bob.last_name}")
Bob Ross
Bob Barker
Bob Marley
Bob Dylan
>>> print(bobs.query)
SELECT "application_employee"."id",
    FROM "application_employee"
    WHERE "application_employee"."first_name" = Bob

What if you want to delete an employee record?

>>> from application.models import Employee
>>> # The price is wrong, Bob!
>>> Employee.objects.filter(first_name='Bob', last_name='Barker').delete()
(1, {'application.Employee': 1})

The QuerySet class has a variety of methods that are useful when working with tables. Some of the methods also have the interesting property of returning a new queryset. This is a beneficial capability when you need to apply additional logic for your query.

from application.models import Employee

employees = Employee.objects.all()  # employees is a QuerySet of all rows!

if should_find_the_bobs:
    employees = employees.filter(first_name='Bob')  # New queryset!

Here’s are some other QuerySet methods that I use constantly:

  • create - As an alternative to creating a record instance and calling save, the manager can create a record directly.
Employee.objects.create(first_name='Bobby', last_name='Tables')
  • get - Use this method when you want one and exactly one record. If your query doesn’t match or will return multiple records, you’ll get an exception.
the_bob = Employee.objects.get(first_name='Bob', last_name='Marley')

# Raises application.models.Employee.MultipleObjectsReturned

Employee.objects.get(first_name='Bob', last_name='Sagat')
# Raises application.models.Employee.DoesNotExist
  • exclude - This method lets you exclude rows that may be part of your existing queryset.
the_other_bobs = (
  • update - With this method, you can update a group of rows in a single operation.
  • exists - Use this method if you want to check if rows exist in the database that match the condition you want to check.
has_bobs = Employee.objects.filter(first_name='Bob').exists()
  • count - Check how many rows match a condition. Because of how SQL works, note that this is more efficient than trying to use len on a queryset.
how_many_bobs = Employee.objects.filter(first_name='Bob').count()
  • none - This returns an empty queryset for the model. How could this be useful? I use this when I need to protect certain data access.
employees = Employee.objects.all()

if not is_hr:
    employees = Employee.objects.none()
  • first / last - These methods will return an individual model instance if one matches. The methods use ordering on the models to get the desired result. We use order_by to tell how we want the results arranged.
>>> a_bob = Employee.objects.filter(first_name='Bob').order_by(
...     'last_name').last()
>>> print(a_bob.last_name)

Types Of Model Data

When we discussed forms, we saw that Django’s form system includes a wide variety of form fields. If you look at the Form fields reference and compare the list of types to those in the Model field reference, you can observe a lot of overlap.

  • default - If you want to be able to create a model record without specifying certain values, then you can use default. The value can either be a literal value or callable function that produces a value.
# application/models.py
import random

from django.db import models

def strength_generator():
    return random.randint(1, 20)

class DungeonsAndDragonsCharacter(models.Model):
    name = models.CharField(max_length=100, default='Conan')
    # Important to note: Pass the function, do not *call* the function!
    strength = models.IntegerField(default=strength_generator)
  • unique - When a field value must be unique for all the rows in the database table, use unique. This is a good attribute for identifiers where you don’t expect duplicates.
class ImprobableHero(models.Model):
    name = models.CharField(max_length=100, unique=True)

# There can be only one.
ImprobableHero.objects.create(name='Connor MacLeod')
  • null - A relational database has the ability to store the absence of data. In the database, this value is thought of as NULL. Sometimes this is an important distinction versus a value that is empty. For instance, on a Person model, an integer field like number_of_children would mean very different things for a value of 0 versus a value of NULL. The former indicates that a person has no children while the latter indicates that the number of children is unknown. The presence of null conditions requires more checking in your code so Django defaults to making null be False. This means that a field does not allow NULL. Null values can be useful if needed, but I think its better to avoid them if you can and try to keep actual data about a field.
class Person(models.Model):
    # This field would always have a value since it can't be null.
    # Zero counts as a value and is not NULL.
    age = models.IntegerField()
    # This field could be unknown and contain NULL.
    # In Python, a NULL db value will appear as None.
    weight = models.IntegerField(null=True)
  • blank - The blank attribute is often used in conjunction with the null attribute. While the null attribute allows a database to store NULL for a field, blank allows form validation to permit an empty field. This is used by forms which are automatically generated by Django like in the Django administrator site which we’ll talk about in the next article.
class Pet(models.Model):
    # Not all pets have tails so we want auto-generated forms
    # to allow no value.
    length_of_tail = models.IntegerField(null=True, blank=True)
  • choices - We saw choices in the forms article as a technique for helping users pick the right value from a constrained set. choices can be set on the model. Django can do validation on the model that will ensure that only particular values are stored in a database field.
class Car(models.Model):
        (1, 'Black'),
        (2, 'Red'),
        (3, 'Blue'),
        (4, 'Green'),
        (5, 'White'),
    color = models.IntegerField(choices=COLOR_CHOICES, default=1)
  • help_text - As applications get bigger or if you work on a large team with many people creating Django models, the need for documentation grows. Django permits help text that can be displayed with a field value in the Django administrator site. This help text is useful to remind your future self or educate a coworker.
class Policy(models.Model):
    is_section_987_123_compliant = models.BooleanField(
            'For policies that only apply on leap days'
            ' in accordance with Section 987.123'
            ' of the Silly Draconian Order'

What Makes A Database “Relational?”

An overly simplified model for an employee with multiple phone numbers might look like:

# application/models.py
from django.db import models

class Employee(models.Model):
    first_name = models.CharField(max_length=100)
    last_name = models.CharField(max_length=100)
    job_title = models.CharField(max_length=200)
    phone_number_1 = models.CharField(max_length=32)
    phone_number_2 = models.CharField(max_length=32)

This single table could hold a couple of numbers, but this solution has some deficiencies.

  • What if an employee has more than two phone numbers? It’s possible for a person to have multiple cell phones, a land line at their residence, a pager number, a fax number, and so on.
  • How can we know what type of phone number is in phone_number_1 and phone_number_2? If you pull the employee record to try to call the individual and dial a fax number instead, you’d have a hard time talking to them.

Instead, what if we had two separate models?

# application/models.py
from django.db import models

class Employee(models.Model):
    first_name = models.CharField(max_length=100)
    last_name = models.CharField(max_length=100)
    job_title = models.CharField(max_length=200)

class PhoneNumber(models.Model):
    number = models.CharField(max_length=32)
        (1, 'Mobile'),
        (2, 'Home'),
        (3, 'Pager'),
        (4, 'Fax'),
    phone_type = models.IntegerField(choices=PHONE_TYPES, default=1)

class PhoneNumber(models.Model):
    number = models.CharField(max_length=32)
        (1, 'Mobile'),
        (2, 'Home'),
        (3, 'Pager'),
        (4, 'Fax'),
    phone_type = models.IntegerField(choices=PHONE_TYPES, default=1)
    employee = models.ForeignKey(Employee, on_delete=models.CASCADE)
# This is what Django adds to your model.
id = models.AutoField(primary_key=True)

An AutoField adds a column to a database table that will assign each row in the table a unique integer. Each new row increments from the previous row and numbering starts at one. This number is the identifier for the row and is called the primary key.

A ForeignKey is a one to many relationship because multiple rows from a table (in this case, PhoneNumber) can reference a single row in another table, namely, Employee. In other words, an employee can have multiple phone numbers. If we wanted to get Tom’s phone numbers, then one possible way would be:

tom = Employee.objects.get(first_name='Tom', last_name='Bombadil')
phone_numbers = PhoneNumber.objects.filter(employee=tom)

The query for phone_numbers would be:

    FROM "application_phonenumber"
    WHERE "application_phonenumber"."employee_id" = 1

There is another relational field type that we should spend time on. That field is the ManyToManyField. As you might guess, this field is used when two types of data relate to each other in a many to many fashion.

What if we tried to model this with ForeignKey fields?

# application/models.py
from django.db import models

class Person(models.Model):
    name = models.CharField(max_length=128)

class House(models.Model):
    address = models.CharField(max_length=256)
    resident = models.ForeignKey(Person, on_delete=models.CASCADE)
# application/models.py
from django.db import models

class House(models.Model):
    address = model.CharField(max_length=256)

class Person(models.Model):
    name = models.CharField(max_length=128)
    house = models.ForeignKey(House, on_delete_models.CASCADE)

Neither of these scenarios model the real world well. In the real world, houses can and do often hold multiple people. Simultaneously, many people in the world have a second house like a beach house or a summer cottage in the woods. Both sides of the model relationship can have many of the other side.

With a ManyToManyField, you can add the field to either side. Here’s the new modeling.

# application/models.py
from django.db import models

class Person(models.Model):
    name = models.CharField(max_length=128)

class House(models.Model):
    address = models.CharField(max_length=256)
    residents = models.ManyToManyField(Person)

Let’s think of an example to see what this looks like. Suppose there are three people records with primary keys of 1, 2, and 3. Let’s also suppose that there are three houses with primary keys of 97, 98, and 99. To prove that the many-to-many relationship works in both directions, we’ll assume these conditions are true:

  • People with primary keys of 1 and 2 reside in house 97.
  • The person with primary key 3 owns house 98 and 99.

The data in the new mapping table between Person and House would contain data like:

Person | House
1      | 97
2      | 97
3      | 98
3      | 99

We can access the many side of each model using a queryset. residents will be a ManyRelatedManager and, like other managers, can provide querysets by using certain manager methods.

house = House.objects.get(address='123 Main St.')
# Note the use of `all()`!
for resident in house.residents.all():

person = Person.objects.get(name='Joe')
for house in person.house_set.all():

Understanding ForeignKey and ManyToManyField is an important step to modeling your problem domain well. By having these tools available to you, you can begin to create many of the complex data relationships that exist with real world problems.


This is how we can use Django models to interact with data. We saw:

  • Saving new information into the database.
  • Asking the database for information that we stored.
  • Complex field types to model real world problems.

Next Time

In the next episode, we are going to explore Django’s built-in administrative tools and look at the Django admin site.

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