In Django, models.py is a Python module that contains the definition of the data models for your application. Each model maps to a database table, and the fields of the model correspond to the columns of the table.

A model is a class that represents a single table in our Database. Django Model is a subclass of django.db.models.Model and each field of the model class represents a database field (column).

Models are defined in a models.py file (example: myapp/models.py)

#models.py

from django.db import models
import uuid
from django.utils.safestring import mark_safe

def uuid_generate():
        return uuid.uuid4().hex

class User(models.Model):
    # Fields
    CHOICES = (
        ('Male', 'Male'),
        ('Female', 'Female'),
    )
    ref_no=models.AutoField(primary_key=True)
    uuid = models.UUIDField(
        primary_key=True, default=uuid.uuid4, editable=False)
    id = models.CharField(
            max_length=32, default=uuid_generate, unique=True, primary_key=True, editable=False)
    first_name = models.CharField(max_length=255, )
    last_name = models.CharField(max_length=255, )
    product=models.IntegerField()
    quantity=models.IntegerField()
    amount=models.DecimalField(decimal_places=2,max_digits=10)
    myfile=models.FileField(upload_to="file")
    picture=models.BinaryField()
    email = models.EmailField(max_length=255,unique=True,verbose_name="Email Address")
    description = models.TextField()
    gender = models.CharField(max_length=255, choices=CHOICES, blank=True, null=True)
    profile = models.ImageField(upload_to="profile", null=True,default="nepal.png")
    #default image must be in media/ directory
    date_joined = models.DateTimeField(('date joined'), auto_now_add=True)
    is_active = models.BooleanField(default=True)
    is_staff = models.BooleanField(default=True)
            created_by = models.ForeignKey(
            'self', on_delete=models.PROTECT, related_name="+", db_column="created_by", null=True)
    created_at = models.DateTimeField(auto_now_add=True)
    updated_at = models.DateTimeField(auto_now=True)

    class Meta:
        verbose_name_plural = "USER"
        ordering = ['name']
        db_table = "user"
        unique_together = ("", "")

    def get_absolute_url(self):
        return reverse('user-view', args=[str(self.id)])
    
    def __str__(self):
        return '%s - %s' % (self.id, self.name)
    
    
    def save(self, *args, **kwargs):
        if self.customer_name == None:
            self.customer_name = datetime.datetime.now()
        super(Customer, self).save(*args, **kwargs)
        if self._state.adding:
            try:
                check_invoice = Invoice.objects.first()
            except Exception:
                check_invoice = None

            if check_invoice:
                last_invoice = Invoice.objects.order_by("invoiceNumber").last()
                last_num = last_invoice.invoiceNumber
            else:
                last_num = 1000

            num = last_num + 1
            self.invoiceNumber = num
        super(Invoice, self).save(*args, **kwargs)


    @property
    def get_full_name(self):
        return f'{self.first_name or ""} {self.last_name or ""}'
    @property
    def price(self):
       return (self.product.price)
    @property
    def amount(self):
       return (self.product.price*self.quantity)

    #to show image in admin panel list
    def image_tag(self):
        return mark_safe('<img src="{}" width="50' height="50"/>'.format(self.profile.url))
    image_tag.short_description="Image"

    #then in admin fille list_display you can mention "image_tag"
    
    def color_by(self):
        return mark_safe('<div style="width:50px;height=50px;background-color:%s"></div>',%(self.color_code)

Once you define models, you need to register your app in INSTALLED_APPS list so that Django know that you are going to use those models.

#settings.py
INSTALLED_APPS = [
#…
"Blog",
#….
]

Onces you add you app in INSTALLED_APPS you need to run two commands makemigrations and migrate. 

makemigrations basically generates the SQL commands for preinstalled apps.

Python manage.py makemigrations

migrate creates the table in the database

Python manage.py migrate

 Additional,  if you want to save the image in specific path then you can custom it like 

#models.py
def path_and_rename(instance,filename):
    upload_to='Images/'
    ext=filename.split(".")[-1] #to get extension
    if instance.user.username:
        filename="user_profile_picture/{}.{}".format(instance.user.username,ext)
        if os.path.exists(filename):
            new_name=str(instance.user.username)+str("1")
            filename="user_profile_picture/{}.{}".format(new_name,ext)
    return os.path.join(upload_to,filename)



#then change, imagefield attribute as
photo=models.ImageField(upload_to=path_and_rename)

In this tutorial we will learn about the additional concept of model in django

Django Model Relationship 

Django provides three ways to define database relationships.

1. One to One Relationship
2. One to Many Relationship
3. Many to Many Relationships