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Object Relational Mapping (ORM) is a technique used in creating a "bridge" between object-oriented programs and, in most cases, relational databases. Put another way, you can see the ORM as the layer that connects object oriented programming (OOP) to relational databases.

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Name		Name	Last commit message	Last commit date
Latest commit History 107 Commits
DataOperationsInDjangoWithQueriesExercise		DataOperationsInDjangoWithQueriesExercise
DataOperationsInDjangoWithQueriesLab		DataOperationsInDjangoWithQueriesLab
DjangoModelsExercise		DjangoModelsExercise
DjangoModelsLab		DjangoModelsLab
DjangoModelsRelationsExercise		DjangoModelsRelationsExercise
DjangoModelsRelationsLab		DjangoModelsRelationsLab
MigrationsAndDjangoAdminExercise		MigrationsAndDjangoAdminExercise
MigrationsAndDjangoAdminLab		MigrationsAndDjangoAdminLab
ModelsInheritanceAndCustomizationExercise		ModelsInheritanceAndCustomizationExercise
ModelsInheritanceAndCustomizationLab		ModelsInheritanceAndCustomizationLab
WorkingWithQueriesInDjangoExercise		WorkingWithQueriesInDjangoExercise
WorkingWithQueriesInDjangoLab		WorkingWithQueriesInDjangoLab
.gitignore		.gitignore
LICENSE		LICENSE
README.md		README.md
image.png		image.png
main.py		main.py

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Python-ORM

ORM - Object Relational Mapping

Django Documentation

Django Extensions Documentation

How to Start Django Project with a Database(PostgreSQL):

Navigate to settings.py
Approximately, in line 76 of code, this is the database config part
Copy the code below, then change it to your corresponding parameters

DATABASES= {"default": {"ENGINE":"django.db.backends.postgresql","NAME":'',"USER":'',"PASSWORD":'',"HOST":"localhost","PORT":"5432",    }}

NAME → Database name e.g. django-project previously created in pgAdmin

USER → Database username (default is postgres)
PASSWORD → Database password
HOST → Database host (In development stage, use localhost or IP Address 127.0.0.1 also available)
PORT → The port that used to run the database (Default port is 5432)

Hide Name, Username, Password and SECRET_KEY

1.Create in your project a file named.env

2.Add this to the file:

DATABASES_NAME='working-queries-django-lab'DATABASES_USER='postgres'DATABASES_PASSWORD='1234'MY_OWN_KEY='django-insecure-cd!p_@ut(kc8)%b_*@)i@kff^oGFrkvy=!c#i!lk9'

3.Change the data with yours in the variables!!!
- DATABASES_NAME
- DATABASES_USER
- DATABASES_PASSWORD
- MY_OWN_KEY

4.After adding the .env file, the database settings should look like this:

SECRET_KEY=config('MY_OWN_KEY')

DATABASES= {"default": {"ENGINE":"django.db.backends.postgresql","NAME":config('DATABASES_NAME'),"USER":config('DATABASES_USER'),"PASSWORD":config('DATABASES_PASSWORD'),"HOST":"localhost","PORT":"5432",    }}

5.In your settings.py file add:
- from decouple import config
6.Run in the Terminal:
- pip install python-decouple

Django Commands:

1. Creating a New Django Project:

django-adminstartproject<projectname>

2. Creating a New Django App:

pythonmanage.pystartapp<appname>

3. Running the Development Server:

pythonmanage.pyrunserver

4. Creating Database Tables:

pythonmanage.pymakemigrations

pythonmanage.pymigrate

5. Creating a Superuser (Admin User):

pythonmanage.pycreatesuperuser

6. Collecting Static Files:

pythonmanage.pycollectstatic

7. Creating a New Migration:

pythonmanage.pymakemigrations<appname>

8. Applying Migrations:

pythonmanage.pymigrate<appname>

9. Creating a New Django User:

pythonmanage.pymigrate<appname>

10. Starting a Django Shell:

pythonmanage.pyshell

11. Creating a Custom Management Command:

pythonmanage.pycreate_commandmy_custom_command

12. Running Tests:

pythonmanage.pytestappname

13. Creating empty migration file:

pythonmanage.pymakemigrations<main_app>--name<migrate_age_group>--empty

14. Edit the Data Migration:

Open the generated data migration file and modify it to useRunPython with a custom Python function.

fromdjango.dbimportmigrationsdefset_age_group(apps,schema_editor):person_model=apps.get_model('main_app','Person')persons=person_model.objects.all()forperson_recordinpersons:ifperson_record.age<=12:person_record.age_group='Child'elifperson_record.age<=17:person_record.age_group='Teen'else:person_record.age_group='Adult'# We save changes to the base once, not every iterationperson_model.objects.bulk_update(persons, ['age_group'])defset_age_group_default(apps,schema_editor):person_model=apps.get_model('main_app','Person')age_group_default=person_model._meta.get_field('age_group').defaultforpersoninperson_model.objects.all():person.age_group=age_group_defaultperson.save()classMigration(migrations.Migration):dependencies= [        ('main_app','0009_person'),    ]operations= [migrations.RunPython(set_age_group,reverse_code=set_age_group_default)    ]

15. Shell debugging:

install:pip install ipython
Run in the Terminal:
```
pythonmanage.pyshell
```

Note: When you run python manage.py shell you run a python (or IPython) interpreter but inside it load all your Django project configurations so you can execute commands against the database or any other resources that are available in your Django project

16. Delete all migrations:

```
pythonmanage.pymigrate<app_name>zero
```
1. Delete all migrations from the 'migrations' folder.
2. python manage.py makemigrations.
3. python manage.py migrate.

17. Debug with ipdb:

pipinstallipdb

We put this where we want to debug:

importipdb;ipdb.set_trace()

Django ORM Commands:

1. Creating a New Model:

 Define your model in the app's models.py file, then create a migration and apply it.

2. Querying the Database:

You can use Django's QuerySet API to retrieve data from the database. For example:

fromappname.modelsimportModelNamedata=ModelName.objects.all()

3. Filtering Data:

data=ModelName.objects.filter(fieldname=value)

4. Creating New Records:

new_record=ModelName(field1=value1,field2=value2)new_record.save()

5. Updating Records:

record=ModelName.objects.get(pk=pk)record.field1=new_valuerecord.save()

6. Deleting Records:

record=ModelName.objects.get(pk=pk)record.delete()

Useful code:

1. Updating the first record in the database:

Model.objects.filter(pk=1).update(is_capital=True)

location=Model.objects.first()location.is_capital=Truelocation.save()

2. Retrieves and returns all records from the database in reverse order:

locations=Model.objects.all().order_by('-id')return'\n'.join(str(x)forxinlocations)

3. Deletes all entries in the database:

Model.objects.all().delete()

4. Deletes the first object:

Model.objects.first().delete()

5. Deletes the last object:

Model.objects.last().delete()

6. Filter by year:

Model.objects.filter(year__gte=2020).values('price','price_with_discount')

Note:Returns records that have a year after 2020. In the QuerySet, show the price and the price with the discount (fields in the database).

7. Filter by Boolean value:

unfinished_task=Model.objects.filter(is_finished=False)return'\n'.join(str(t)fortinunfinished_task)

Note:Filters by boolean value. Returns all records for which the is_finished field in the database is False.

8. Filter by odd IDs:

fortaskinModel.objects.all():iftask.id%2!=0:task.is_finished=Truetask.save()

Note:Loops through all objects in the database of the given model.Then it checks if the id is odd. If so, set the field is_finished=True.

9. Decodes and replaces the text:

Оptimized solution:

defdecodes_and_replace(text:str,task_title:str)->None:decoded_text=''.join(chr(ord(x)-3)forxintext)# -> Wash the dishes!Model.objects.filter(title=task_title).update(description=decoded_text)encode_and_replace("Zdvk#wkh#glvkhv$","Simple Task")

Non-optimized solution:

tasks_with_matching_title=Model.objects.filter(title=task_title)decoded_text=''.join(chr(ord(x)-3)forxintext)fortaskintasks_with_matching_title:task.description=decoded_texttask.save()encode_and_replace("Zdvk#wkh#glvkhv$","Simple Task")

Before:

After:

Note:Filters by title "Simple task", changes description to "Zdvk#wkh#glvkhv$", which decodes to "Wash the dishes!'

10. Update characteres:

# If the class name is "Mage" - increase the level by 3 and decrease the intelligence by 7.Model.objects.filter(calss_name="Mage").update(level=F('level')+3,intelligence=F('intelligence')-7,)# If the class name is "Warrior" - decrease the hit points by half and increase the dexterity by 4.Model.objects.filter(class_name="Warrior").update(hit_points=F('hit_points')/2,dexterity=F('dexterity')+4)# If the class name is "Assassin" or "Scout" - update their inventory to "The inventory is empty".Model.objects.filter(class_name__in=["Assassin","Scout"]).update(inventory="The inventory is empty",)

Note:An F() object represents the value of a model field, transformed value of a model field, or annotated column. It makes it possible to refer to model field values and perform database operations using them without actually having to pull them out of the database into Python memory. Instead, Django uses the F() object to generate an SQL expression that describes the required operation at the database level.

11. Update a field in the database of a specific model. With SQL query and Python ORM on database column named "class_name" with value "Mage":

SQL query:UPDATE<model>SET level= level+3WHERE class_name="Mage";  Python ORM:Model.objects.filter(calss_name="Mage").update(            level=F('level')+3,        )

12. Filter by two parameters:

best_artwork=Model.objects.order_by('-rating','id').first()

'-rating': Filter by the 'rating' field and take the entry with the highest rating.
'id': If all records have the same rating then filter and take the record with the highest 'id'.

13. Deletes all objects that have a negative rating. 0 (zero) is considered positive:

Model.objects.filter(rating__lt=0).delete()

rating__lt < 0 (delete all entries below zero)
ratin <= 0 (delete all entries below zero including zero)

14. Update the storage of the brand if it is 'Lenovo' or 'Asus':

Model.objects.filter(Q(brand='Lenovo')|Q(brand='Asus')).update(storage=512)

Model.objects.filter(brand_in=['Lenovo','Asus']).update(storage=512)

Note:The database field names are 'brand' and 'storage.

15. Updates the operation system for every laptop:

Laptop.objects.filter(brand=['Asus']).update(operation_system='Windows')Laptop.objects.filter(brand=['Apple']).update(operation_system='MacOS')Laptop.objects.filter(brand__in=['Dell','Acer']).update(operation_system='Linux')Laptop.objects.filter(brand=['Lenovo']).update(operation_system='Chrome OS')

Optimization:

Laptop.objects.update(operation_system=Case(When(brand=['Asus'],then=Value('Windows')),When(brand=['Apple'],then=Value('MacOS')),When(brand=['Dell','Acer'],then=Value('Linux')),When(brand=['Lenovo'],then=Value('Chrome OS')),default=F('operation_system')    ))

We import them from django:Value(), When(), F()

Note:It checks the laptop brand and records the specific operating system.

16. Exlude:

Model.objects.exlude(difficulty='Easy').update(boss_health=500)

Note:Exclude all entries except entry with difficulty 'Easy'. And updated it with boss_health=500.

17. LOGGING:

In your settings.py file add:

LOGGING= {'version':1,'disable_existing_loggers':False,'handlers': {'console': {'class':'logging.StreamHandler'}},'root': {'handlers': ['console'],'level':'DEBUG',    },'loggers': {'django.db.backends': {'handlers': ['console'],'level':'DEBUG','propagate':False,}}  }

Django Models Relations

One-To-Many Relationship

classLecturer(models.Model):first_name=models.CharField(max_length=100)last_name=models.CharField(max_length=100)def__str__(self):returnf"{self.first_name}{self.last_name}"# manyclassSubject(models.Model):name=models.CharField(max_length=100)code=models.CharField(max_length=100)# One-To-Many Relationshiplecturer=models.ForeignKey('Lecturer',on_delete=models.SET_NULL,null=True)def__str__(self):returnf"{self.name}"

Note: Lecturer can have many subjects. One subject can have one lecturer.

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Python-ORM

How to Start Django Project with a Database(PostgreSQL):

Hide Name, Username, Password and SECRET_KEY

Django Commands:

1. Creating a New Django Project:

2. Creating a New Django App:

3. Running the Development Server:

4. Creating Database Tables:

5. Creating a Superuser (Admin User):

6. Collecting Static Files:

7. Creating a New Migration:

8. Applying Migrations:

9. Creating a New Django User:

10. Starting a Django Shell:

11. Creating a Custom Management Command:

12. Running Tests:

13. Creating empty migration file:

14. Edit the Data Migration:

15. Shell debugging:

16. Delete all migrations:

17. Debug with ipdb:

Django ORM Commands:

1. Creating a New Model:

2. Querying the Database:

You can use Django's QuerySet API to retrieve data from the database. For example:

3. Filtering Data:

4. Creating New Records:

5. Updating Records:

6. Deleting Records:

Useful code:

1. Updating the first record in the database:

2. Retrieves and returns all records from the database in reverse order:

3. Deletes all entries in the database:

4. Deletes the first object:

5. Deletes the last object:

6. Filter by year:

7. Filter by Boolean value:

8. Filter by odd IDs:

9. Decodes and replaces the text:

Оptimized solution:

Non-optimized solution:

Before:

After:

10. Update characteres:

11. Update a field in the database of a specific model. With SQL query and Python ORM on database column named "class_name" with value "Mage":

12. Filter by two parameters:

13. Deletes all objects that have a negative rating. 0 (zero) is considered positive:

14. Update the storage of the brand if it is 'Lenovo' or 'Asus':

15. Updates the operation system for every laptop:

Optimization:

16. Exlude:

17. LOGGING:

Django Models Relations

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