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unique=True – Django Built-in Field Validation

  • Last Updated : 13 Feb, 2020

Built-in Field Validations in Django models are the validations that come predefined to all Django fields. Every field comes in with built-in validations from Django validators. One can also add more built-in field validations for applying or removing certain constraints on a particular field. unique=True sets the field to be unique i.e. once entered a value in a field, the same value can not be entered in any other instance of that model in any manner. It is generally used for fields like Roll Number, Employee Id, etc which should be unique.

Syntax

field_name = models.Field(unique=True)

Django Built-in Field Validation unique=True Explanation

Illustration of unique using an Example. Consider a project named geeksforgeeks having an app named geeks.

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

Enter the following code into models.py file of geeks app. We will be using CharField for experimenting for all field options.






from django.db import models
from django.db.models import Model
# Create your models here.
  
class GeeksModel(Model):
    geeks_field = models.CharField(
                    max_length = 200,  
                    unique = True
                    )

After running makemigrations and migrate on Django and rendering the above model, let us create an instance from Django admin interface with string “a“. Now to show the constraint of unique=True, let us try to create one more instance of the model using same string. Now it will show this error.

django-error-messages

Advanced concepts with unique

This is enforced at the database level and by model validation. If you try to save a model with a duplicate value in a unique field, a django.db.IntegrityError will be raised by the model’s save() method.This option is valid on all field types except ManyToManyField and OneToOneField.
Note that when unique is True, you don’t need to specify db_index, because unique implies the creation of an index.

More Built-in Field Validations

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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