Модели Django

Опубликовано: 24 Февраля, 2022

Модель Django - это встроенная функция, которую Django использует для создания таблиц, их полей и различных ограничений. Короче говоря, Django Models - это SQL базы данных, который используется с Django. SQL (язык структурированных запросов) сложен и включает в себя множество различных запросов для создания, удаления, обновления или любых других вещей, связанных с базой данных. Модели Django упрощают задачи и организуют таблицы в модели. Как правило, каждая модель отображается в одну таблицу базы данных.
Эта статья посвящена тому, как можно использовать модели Django для удобного хранения данных в базе данных. Более того, мы можем использовать панель администратора Django для создания, обновления, удаления или получения полей модели и различных подобных операций. Модели Django обеспечивают простоту, согласованность, контроль версий и расширенную обработку метаданных. Основы модели включают -

  • Each model is a Python class that subclasses django.db.models.Model.
  • Каждый атрибут модели представляет собой поле базы данных.
  • With all of this, Django gives you an automatically-generated database-access API; see Making queries.

    Example –

    from django.db import models
      
    # Create your models here.
    class GeeksModel(models.Model):
        title = models.CharField(max_length = 200)
        description = models.TextField()

    Django maps the fields defined in Django models into table fields of the database as shown below.

    Using Django Models

    To use Django Models, one needs to have a project and an app working in it. After you start an app you can create models in app/models.py. Before starting to use a model let’s check how to start a project and create an app named geeks.py

    Refer to the following articles to check how to create a project and an app in Django.

    • How to Create a Basic Project using MVT in Django?
    • How to Create an App in Django ?

    Creating a Model

    Syntax

    from django.db import models
            
    class ModelName(models.Model):
            field_name = models.Field(**options)
    

    To create a model, in geeks/models.py Enter the code,

    # import the standard Django Model
    # from built-in library
    from django.db import models
      
    # declare a new model with a name "GeeksModel"
    class GeeksModel(models.Model):
            # fields of the model
        title = models.CharField(max_length = 200)
        description = models.TextField()
        last_modified = models.DateTimeField(auto_now_add = True)
        img = models.ImageField(upload_to = "images/")
      
            # renames the instances of the model
            # with their title name
        def __str__(self):
            return self.title

    Whenever we create a Model, Delete a Model, or update anything in any of models.py of our project. We need to run two commands makemigrations and migrate. makemigrations basically generates the SQL commands for preinstalled apps (which can be viewed in installed apps in settings.py) and your newly created app’s model which you add in installed apps whereas migrate executes those SQL commands in the database file.
    So when we run,

    Python manage.py makemigrations

    SQL Query to create above Model as a Table is created and

     Python manage.py migrate

    creates the table in the database.

    Now we have created a model we can perform various operations such as creating a Row for the table or in terms of Django Creating an instance of Model. To know more visit – Django Basic App Model – Makemigrations and Migrate

    Render a model in Django Admin Interface

    To render a model in Django admin, we need to modify app/admin.py. Go to admin.py in geeks app and enter the following code. Import the corresponding model from models.py and register it to the admin interface.

    from django.contrib import admin 
        
    # Register your models here. 
    from .models import GeeksModel 
        
    admin.site.register(GeeksModel) 

    Now we can check whether the model has been rendered in Django Admin. Django Admin Interface can be used to graphically implement CRUD (Create, Retrieve, Update, Delete).

    To check more on rendering models in django admin, visit –Render Model in Django Admin Interface



    Django CRUD – Inserting, Updating and Deleting Data

    Django lets us interact with its database models, i.e. add, delete, modify and query objects, using a database-abstraction API called ORM(Object Relational Mapper). We can access the Django ORM by running the following command inside our project directory.

    python manage.py shell

    Adding objects.
    To create an object of model Album and save it into the database, we need to write the following command:

    >>> a = GeeksModel(
             title = “GeeksForGeeks”,  
             description = “A description here”,
             img = “geeks/abc.png”
             )
    >>> a.save()
    

    Retrieving objects
    To retrieve all the objects of a model, we write the following command:

    >>> GeeksModel.objects.all()
    <QuerySet [<GeeksModel: Divide>, <GeeksModel: Abbey Road>, <GeeksModel: Revolver>]>
    

    Modifying existing objects
    We can modify an existing object as follows:

    >>> a = GeeksModel.objects.get(id = 3)
    >>> a.title = "Pop"
    >>> a.save()
    

    Deleting objects
    To delete a single object, we need to write the following commands:

    >>> a = Album.objects.get(id = 2)
    >>> a.delete()
    

    To check detailed post of Django’s ORM (Object) visit Django ORM – Inserting, Updating & Deleting Data

    Validation on Fields in a Model

    Built-in Field Validations in Django models are the default validations that come predefined to all Django fields. Every field comes in with built-in validations from Django validators. For example, IntegerField comes with built-in validation that it can only store integer values and that too in a particular range.
    Enter the following code into models.py file of geeks app.

    from django.db import models
    from django.db.models import Model
    # Create your models here.
      
    class GeeksModel(Model):
        geeks_field = models.IntegerField()
      
        def __str__(self):
            return self.geeks_field

    After running makemigrations and migrate on Django and rendering above model, let us try to create an instance using string “GfG is Best“.

    You can see in the admin interface, one can not enter a string in an IntegerField. Similarly every field has its own validations. To know more about validations visit, Built-in Field Validations – Django Models

    More on Django Models –

    • Change Object Display Name using __str__ function – Django Models
    • Custom Field Validations in Django Models
    • Django python manage.py migrate command
    • Django App Model – Python manage.py makemigrations command
    • Django model data types and fields list
    • How to use Django Field Choices ?
    • Overriding the save method – Django Models

    Basic model data types and fields list

    The most important part of a model and the only required part of a model is the list of database fields it defines. Fields are specified by class attributes. Here is a list of all Field types used in Django.

    Field NameDescription
    AutoFieldIt An IntegerField that automatically increments.
    BigAutoFieldIt is a 64-bit integer, much like an AutoField except that it is guaranteed to fit numbers from 1 to 9223372036854775807.
    BigIntegerFieldIt is a 64-bit integer, much like an IntegerField except that it is guaranteed to fit numbers from -9223372036854775808 to 9223372036854775807.
    BinaryFieldA field to store raw binary data.
    BooleanFieldA true/false field.
    The default form widget for this field is a CheckboxInput.
    CharFieldIt is a date, represented in Python by a datetime.date instance.
    DateFieldA date, represented in Python by a datetime.date instance
    It is used for date and time, represented in Python by a datetime.datetime instance.DecimalFieldIt is a fixed-precision decimal number, represented in Python by a Decimal instance.DurationFieldA field for storing periods of time.EmailFieldIt is a CharField that checks that the value is a valid email address.FileFieldIt is a file-upload field.FloatFieldIt is a floating-point number represented in Python by a float instance.ImageFieldIt inherits all attributes and methods from FileField, but also validates that the uploaded object is a valid image.IntegerFieldIt is an integer field. Values from -2147483648 to 2147483647 are safe in all databases supported by Django.GenericIPAddressFieldAn IPv4 or IPv6 address, in string format (e.g. 192.0.2.30 or 2a02:42fe::4).NullBooleanFieldLike a BooleanField, but allows NULL as one of the options.PositiveIntegerFieldLike an IntegerField, but must be either positive or zero (0).PositiveSmallIntegerFieldLike a PositiveIntegerField, but only allows values under a certain (database-dependent) point.SlugFieldSlug is a newspaper term. A slug is a short label for something, containing only letters, numbers, underscores or hyphens. They’re generally used in URLs.SmallIntegerFieldIt is like an IntegerField, but only allows values under a certain (database-dependent) point.TextFieldA large text field. The default form widget for this field is a Textarea.TimeFieldA time, represented in Python by a datetime.time instance.URLFieldA CharField for a URL, validated by URLValidator.UUIDFieldA field for storing universally unique identifiers. Uses Python’s UUID class. When used on PostgreSQL, this stores in a uuid datatype, otherwise in a char(32).

    Relationship Fields

    Django also defines a set of fields that represent relations.

    Field NameDescription
    ForeignKeyA many-to-one relationship. Requires two positional arguments: the class to which the model is related and the on_delete option.
    ManyToManyFieldA many-to-many relationship. Requires a positional argument: the class to which the model is related, which works exactly the same as it does for ForeignKey, including recursive and lazy relationships.
    OneToOneFieldA one-to-one relationship. Conceptually, this is similar to a ForeignKey with unique=True, but the “reverse” side of the relation will directly return a single object.

    Field Options

    Field Options are the arguments given to each field for applying some constraint or imparting a particular characteristic to a particular Field. For example, adding an argument null = True to CharField will enable it to store empty values for that table in relational database.
    Here are the field options and attributes that an CharField can use.

    Field OptionsDescription
    NullIf True, Django will store empty values as NULL in the database. Default is False.
    BlankIf True, the field is allowed to be blank. Default is False.
    db_columnThe name of the database column to use for this field. If this isn’t given, Django will use the field’s name.
    DefaultThe default value for the field. This can be a value or a callable object. If callable it will be called every time a new object is created.
    help_textExtra “help” text to be displayed with the form widget. It’s useful for documentation even if your field isn’t used on a form.
    primary_keyIf True, this field is the primary key for the model.
    editableIf False, the field will not be displayed in the admin or any other ModelForm. They are also skipped during model validation. Default is True.
    error_messagesThe error_messages argument lets you override the default messages that the field will raise. Pass in a dictionary with keys matching the error messages you want to override.
    help_textExtra “help” text to be displayed with the form widget. It’s useful for documentation even if your field isn’t used on a form.
    verbose_nameA human-readable name for the field. If the verbose name isn’t given, Django will automatically create it using the field’s attribute name, converting underscores to spaces.
    validatorsA list of validators to run for this field. See the validators documentation for more information.
    UniqueIf True, this field must be unique throughout the table.

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