Django ORM Tutorial

What is a QuerySet?

QuerySet is, in essence, a list of objects of a given Model. QuerySets allow you to read the data from the database, filter it and order it.

What is an ORM?

ORM is an acronym for the object-relational mapper. Django ORM provides an elegant and powerful way to interact with the database. The ORM’s main goal is to transmit data between a relational database and application model.

Example:

from django.db import models
class Student(models.Model):
       surname = models.CharField(max_length=200)
       firstname = models.CharField(max_length=200)
       other_name = models.TextField(blank=True)
       gender = models.TextField(default='male')
       date = models.DateField(auto_now=True)

Note: auto_now – updates the value of field to current time and date every time the Model. save() is called. auto_now_add – updates the value with the time and date of creation of record

Let’s start by creating an Student object:

from app.models import Student
a = Student(surname ='jerry', firstname ='tom', other_name ='tj', gender="male")
a.save() 

• To save the object to the database call the save() method.
• save() method comes from models.Model class.

Let’s delete the object from the database:

if you want to delete one instance then write the code

entry= Account.objects.get(id=5)
entry.delete()

if you want to delete all instance then write the code

entries= Account.objects.all()
entries.delete()

Defining str() method on model

str() is a special method which tells Python how to display an object in human readable form

class Student(models.Model):
       surname = models.CharField(max_length=200)
       firstname = models.CharField(max_length=200)
       other_name = models.TextField(blank=True)
       gender = models.TextField(default='male')
       date = models.DateField(auto_now=True)
       def __str__(self):
             return self.firstname + " : " + self.other_name

we run makemigrations command only in the following two cases:

• When we add/modify fields in the model.
• When we adding/modify Meta classes.

create() method:

create() method allows us to create and commit object to the database. instead of separately calling the save() method.

Student.objects.create(surname="jan", firstname="tom", other_name="",gender="male", date="")

bulk_create() method:

bulk_create() method create and commit multiple objects at a time.

Student.objects.bulk_create([
Student(surname="jan", firstname="tom", other_name="",gender="male", date=""),
Student(surname="jan1", firstname="tom1", other_name="",gender="female", date="")
])

all() method:

all() method fetches all the records from specified table.

Student.objects.all()

count() method:

count() method returns the total number of records in a database table.

Student.objects.all().count()
Student.objects.filter(firstname="john").count()
Student.objects.count()

filter() method:

filter() method returns a subset of data. If we had two records where gender is ‘male’ then filter() would have returned a QuerySet object containing two Author objects.

Student.objects.filter(gender='male')

Django Field Lookups

contains lookup:

• __contains lookup is case-sensitive
• __icontains lookup is case-insensitive

Student.objects.filter(firstname__contains="te")

startswith lookup:

• __startswith lookup finds whose name field start with “j”
• __istartswith lookup is case-insensitive

Student.objects.filter(firstname__startswith="j")

endswith lookup:

• __endswith lookup finds whose name field start with “j”
• __iendswith lookup is case-insensitive

Student.objects.filter(firstname__endswith="ru")

gt and lt lookup:

Student.objects.filter(id__gt=3)
Student.objects.filter(id__lt=3)

# greater than or equal to
Student.objects.filter(id__gte=3)
Student.objects.filter(id__lte=3)

get() method:

get() method returns only single object. If it finds multiple objects it raises a MultipleObjectsReturned exception. If it doesn’t find any object it raises DoesNotExist exception.

Student.objects.get(id=1000)

Ordering Results:

Student.objects.order_by("id") #ASC order
Student.objects.order_by("-id") #DESC order

Selecting the fields:

values_list() method returns a QuerySet where each element is a tuple.

Student.objects.values_list("id", "firstname")

Output:
QuerySet [(1, 'mickle'), (2, 'tom')]

values(): it returns a QuerySet where each element is a dictionary.

Student.objects.filter(id__gt=3).values("id", "name")

Output:
QuerySet [{'id': '4', 'firstname': 'mickle'}, {'id': 5,  'firstname': 'john'}]

Slicing Results

Student.objects.order_by('-id')[1] # returns the 2nd record after sorting the results

Updating Objects

First way:
a = Author.objects.get(id=2)
print(a.firstname) => "john"
a.firstname = "tom"
a.save()

second way:
Student.objects.filter(id=2).update(email='tom@gmail.com')

Deleting records

delete() method is used to delete one or more records.

a = Student.objects.get(pk=2)
a.delete()   #to delete specified record

b = Student.objects.all()
b.delete() #to delete multiple records