Models in DjangoCourtesy: djangoBook.com

Haris NPharis@baabtra.comwww.facebook.com/haris.np9twitter.com/np_harisin.linkedin.com/in/harisnp

MVC v/s MTV

• Model – Data access layer. • View – Presentation Layer. What the user sees. • Controller – Business Logic Layer. This layer

decides which view to use based on the user input and which model to access.

MTV

• M stands for Model• T stands for Template. This is presentation

layer. • V stands for View. It is different from the MVC

architecture. It contains the business logic layer. It access the model and calls the Template as per the clicks.

Connecting to the database• Settings.py

DATABASES = { 'default': { 'ENGINE': 'django.db.backends.mysql', # Add 'postgresql_psycopg2', 'mysql', 'sqlite3' or 'oracle'. 'NAME': 'baabtra', # Or path to database file if using sqlite3. This is the name of the database. # The following settings are not used with sqlite3: 'USER': 'root', 'PASSWORD': 'itsbaabtra', 'HOST': 'localhost', # Empty for localhost through domain sockets or '127.0.0.1' for localhost through TCP. 'PORT': '3306', # Set to empty string for default. }}

Screenshot of Database properties from settings.py page (mysql)

Check the connectivity

• Go to the command prompt • Traverse to the project folder Type :

python manage.py shell

• Type – from django.db import connection– cursor = connection.cursor()If there is no error, then you have configured it correctly.

Project v/s App in Django

• Project: Set of multiple apps with its configuration.

• App: Set of Django functionality. Apps are portable and reusable across multiple projects.

• For starting an app, please use the following command

python manage.py startapp nameoftheapp

• python manage.py startapp baabtramodel

INSTALLED_APP

• Models– It is the description of the data in your database. – It is written in python code. – It is equivalent to CREATE TABLE. If the table doesn’t exist,

when the project is synched with the database that you are creating, the tables are created. If you want to migrate your application from MySQL to Postgres, you don’t have to rewrite the SQL codes again.

– Since the SQL Codes are written in the python, version control of database is easy. If there are modifications to the existing database, it might create out of sync problems.

• Sample Modelclass Category(models.Model): category_code=models.CharField(max_length=10) category_description=models.CharField(max_length=300) def __unicode__(self): return self.category_codeclass Item(models.Model): item_name=models.CharField(max_length=200) item_code=models.CharField(max_length=10) item_barcode=models.CharField(max_length=20) item_description=models.CharField(max_length=300) item_cost=models.FloatField(max_length=10) item_retail_price=models.FloatField(max_length=10) item_category_id=models.ForeignKey(Category) def __unicode__(self): return self.item_name class Supplier(models.Model): supplier_name=models.CharField(max_length=50) supplier_code=models.CharField(max_length=10) credit_period=models.CharField(max_length=50) credit_limit=models.FloatField(max_length=10)class Purchase_order(models.Model): supplier_code=models.FloatField(max_length=15) entry_date=models.DateField() order_amount=models.FloatField(max_length=10) net_amount=models.FloatField(max_length=15) document_type=models.CharField(max_length=20)class Purchase_items(models.Model): order_id=models.FloatField(max_length=15) item_id=models.FloatField(max_length=15) item_quantity=models.FloatField(max_length=10) item_rate=models.FloatField(max_length=10) total_amount=models.FloatField(max_length=15)

• Now run the synchDB command using the following command:

• Run python manage.py validate . This command validates the model.

• python manage.py syncdb

• Basic data access– Go to python manage.py shell– Type the following

>>> from baabtramodel.models import Category>>> obj_category = Category( category_code = 'Elect', category_description ='All kind of electronics gadgets')>>> obj_category .save()Please note that the above code is used for inserting to database table without foreign key reference

• Data is saved in the database. You can verify by going to the database as shown below.

• Adding two more category objects>>> obj_category2 = Category( category_code = 'Cod2', category_description ='category desc2')>>> obj_category3 = Category( category_code = 'cod3', category_description ='category desc3')>>> obj_category2.save()>>> obj_category3.save()

• List the objects>>> category_list = Category.objects.all()>>> category_listIt lists all the objects along with the code name. It

• Reason: Copy pasting the model snippet below

class Category(models.Model): category_code=models.CharField(max_length=10) category_description=models.CharField(max_length=300) def __unicode__(self): return self.category_code

• The __unicode__(self) is playing the trick. A __unicode__() method tells Python how to display the “unicode” representation of an object. As we have given category_code, we will get Elect, Cod2 and Cod3 when the objects are listed.

[<Category: Elect>, <Category: Cod2>, <Category: cod3>]

• You can add multiple strings to a unicode. def __unicode__(self): return u'%s %s' % (self.category_code, self.category_description)

Then it will print both category code and description as shown below.

• Updating data in the database. >>> obj_category2 = Category( category_code = 'Cod2', category_description ='category desc2')>>> obj_category2 .save()>>> obj_category2 .id7 # Depending upon your entry in the table it can defer.

• From the database

>>> obj_category2.category_code= 'code‘>>> obj_category2.save()

Again from the database

Please note that cod2 has been changed to ‘code’

Selecting data from the database

• Select * from baabtramodel_category can be achieved by the following code

• Category.objects.all() #where Category is the class name in the model.

• Filtering data>>> Category.objects.filter(category_code='Elect')

>>>Category.objects.filter(category_code='Elect‘, category_description=‘All kind of electronics gadgets’)

Now change the criteria as >>>Category.objects.filter(category_code='Elect', category_description='All kind of electronics gadget') # doesn’t exist any row in the table which satisfy the criteria

• SQL “like”>>>Category.objects.filter(category_code__contains ='cod')

Please note that it is case sensitive. Data in the database

Retrieving single objects

>>> Category.objects.get(id=7)>>> Category.objects.get(category_code =‘code’) A query that returns more than two or no records causes an exceptionError: Traceback (most recent call last): File "<console>", line 1, in <module> File "C:\Python27\lib\site-packages\django\db\models\manager.py", line 143, in get return self.get_query_set().get(*args, **kwargs) File "C:\Python27\lib\site-packages\django\db\models\query.py", line 407, in get (self.model._meta.object_name, num))MultipleObjectsReturned: get() returned more than one Category -- it returned 2!

Order by and chaining look ups

For descending>>>Category.objects.filter(category_code__contains ='cod').order_by("category_description") For ascending>>>Category.objects.filter(category_code__contains ='cod').order_by(“category_description”)

Slicing of Data

>>>Category.objects.order_by('category_code')[0]Returns only one row. >>>Category.objects.order_by('category_code')[0:2]

Updating multiple rows

>>> Category.objects.all().update (category_code='CodeU')SQL View:

Inserting data in Django to tables with foreign key reference.

>>> from baabtramodel.models import Item>>> obj_item = Item( item_name= 'pen', item_code='p', item_barcode ='bc001', item_description ='Used for writing', item_cost = 10, item_retail_price =15 , item_category_id =1)

• Error• Traceback (most recent call last):– File “<console>”, line 1, in <module>– File “C:\Python27…. base.py”, line 403, in __init__• setattr(self, field.name, rel_obj)

– File “C:\Python27….related.py”, line 405, in __set__• Self.field_name, self.field.rel.to._meta.object_name)

– ValueError: Cannot assign “1”: “Item.item_category_id” must be a “Category” instance.

Fix• Create an instance of the Category. >>> from baabtramodel.models import Category>>> category_id = Category.objects.get(id=1)>>> from baabtramodel.models import Item>>> obj_item = Item( item_name= 'pen', item_code='p', item_barcode ='bc001', item_description ='Used for writing', item_cost = 10, item_retail_price =15 , item_category_id =category_id )Please note that category_id is an instance and not the variable inside the object.

• From a sample application

views.py

• Please note that we are passing categoryid directly here. It must be changed to an instance.

Updated views.py

• Please note that Id is an instance of the class Category

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