Relationships

• In relational database, there can be the following types of relations

  • one to one
  • Many to one
  • Many to Many

• Many to One:

  • Consider the data about employees belonging to different departments stored in a table employee with their employee ID as primary key alongside a column that stores a department, this table also contains a colum that stores departments deopartment id.

  class Department(models.Model):
      name = models.CharField(max_length=50)
      description = models.CharField(max_length=255)
  
  class Employee(models.Model):
      name = models.CharField(max_length=50)
      email = models.EmailField(help_text="Email id of the employee")
      department = models.ForeignKey(Department, on_delete=models.CASCADE)
  
  

  • Many to Many:
    • models.ManyToManyField
  • One to One:
    • Python django has models.OneToOneField

Migration Commands

• Make migrations which would create a file in migrations folder

python manage.py makemigrations <appname>

• to view the migrations

python manage.py showmigrations

• To understand the sql equivalent commands generated by migration

python manage.py sqlmigrate <app_name> <generated_file>
python manage.py sqlmigrate books 0001_inital

• To apply the changes to the database

python manage.py migrate <appname>

Django’s Database CRUD Operations

• Django ORM provides the capability to deal with the CRUD operations without dealing SQL Statements.
• To do this lets enter the python Django’s shell
• Execute python manage.py shell
• Now view the table in DB Browser
• Now try to write the following in the shell

Publisher.objects.get()
Publisher.objects.all()