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Django - Model

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• Django - Model
  • Model
  • Migration
  • Example: Create model and migrate
    • Create model class: models.py
    • Register Model: settings.py
    • Create Migration Files: makemigrations
    • Update Database: migrate
    • List Migrations Files(Opetional): showmigrations
    • Prit SQL Code(Optional): sqlmigrate
  • Model Class
    • Exception
    • Attributes
    • Instance method

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Model

• model:

  • a class used to contain essential fields and methods.
  • Each model class maps to a single table in the database.
  • a subclass of django.db.models.Model and each field of the model class represents a database field (column).

• Model is defined in models.py file of each application package.

• Primary key fields

  • Each model requires exactly one field to have primary_key=True (either explicitly declared or automatically added).

    • By default, Django gives each model an auto-incrementing primary key.
    • If specify a custom primary key, specify primary_key=True on one of fields.
    • If one of fields is explicitly set Field.primary_key, django won’t add the automatic id column.

• SQL

  • Table Name:

    • automatically derived from some model metadata
    • can be overridden.

  • CREATE TABLE SQL:

    • Django uses SQL tailored to the database backend specified in settings file.

• objects attribure:

  • the default name of Manager instance, the interface through which Django models take database query operations.
  • used to retrieve the instances from the database.

• Model Methods

  • Define custom methods on a model to add custom “row-level” functionality to objects.
  • a valuable technique for keeping business logic in one place – the model.

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Migration

• Migration:

  • a way of applying changes that have been made to a model, into the database schema.

• Django creates a migration file inside the migration folder for each model to create the table schema, and each table is mapped to the model of which migration is created.

• Command to perform migration:

Command	Description
makemigrations	create a migration py file for the tabled schema of a model.
migrate	create table according to the schema defined in the migration file.
sqlmigrate	show a raw SQL query of the applied migration.
showmigrations	lists out all the migrations and their status.

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Example: Create model and migrate

Create model class: models.py

• Create a new class which inherits from models.Model

• <app_name> / models.py:

from django.db import models

class Employee(models.Model):
    first_name = models.CharField(max_length=32)
    last_name = models.CharField(max_length=32)
    email = models.EmailField()

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Register Model: settings.py

• Register model into the INSTALLED_APPS inside settings.py.

• settings.py:

INSTALLED_APPS = [
  #...
  'app_name',
  #...
]

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Create Migration Files: makemigrations

• Creates new migrations based on the changes detected to models.

• CLI:

# create migration files for all apps
$ py manage.py makemigrations

# create migrations files for a specific app
$ py manage.py makemigrations app_label1 app_label2 ...

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Update Database: migrate

• migrate
  • Synchronizes the database state with the current set of models and migrations.

$ py manage.py migrate [app_label] [migration_name]

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List Migrations Files(Opetional): showmigrations

• showmigrations

  • Shows all migrations in a project

• CLI:

# show migrations for all apps
$ py manage.py showmigrations

# show migrations for specific apps
$ py manage.py showmigrations [app_label]

# return:
# EmpApp
#  [X] 0001_initial

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Prit SQL Code(Optional): sqlmigrate

• sqlmigrate
  • Prints the SQL for the named migration.
  • This requires an active database connection, which it will use to resolve constraint names;

$ py manage.py sqlmigrate app_label migration_name
# app_label: the name of current application
# migration_name: the name of migration file


# example:
$ py manage.py sqlmigrate EmpApp 0001
# return:
# BEGIN;
# --
# -- Create model Employee
# --
# CREATE TABLE "EmpApp_employee" ("id" integer NOT NULL PRIMARY KEY AUTOINCREMENT, "first_name" varchar(32) NOT NULL, "last_name" varchar(32) NOT NULL, "email" varchar(254) NOT NULL);
# COMMIT;

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Model Class

• django.db.models.Model

Exception

Exception	Description
Model.DoesNotExist	when an expected object is not found
Model.MultipleObjectsReturned	when multiple objects are found

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Attributes

Attributes	Description
Model.objects	the default name of the Manager

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Instance method

• Contructor

Method	Description
Model(**kwargs)	instantiatie a model without db

• Refereshing Object

Method	Description
refresh_from_db()	reload a model’s values from the database
get_deferred_fields()	return the attribute names of all fields

• Validating

Method	Description
clean_fields()	validate all fields on model
clean()	custom model validation
validate_unique()	validates uniqueness constraints
validate_constraints()	validates all constraints
full_clean()	call the above methods

• Saving objects

Method	Description
save()	insert / udpate an object back to the database
delete()	deletes the object in the database

• Helping Method

Method	Description
__str__()	display a model instance
__eq__()	compare primary key value
get_absolute_url()	return a HTTP string to refer to the object
get_field_name_display()	returns the “human-readable” value of the choice field

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