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. error_messages attribute is used to modify error messages that are displayed in the admin interface during failure of some constraint.
For example, you can override “This field is required” message to your own message. It 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. Error message keys include null, blank, invalid, invalid_choice, unique, and unique_for_date.
Syntax –
field_name = models.Field(error_messages = {"key": "message"})
Django Built-in Field Validation editable=False Explanation
Illustration of error_messages using an Example. Consider a project named neveropen having an app named Lazyroar.
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 Lazyroar app. We will be using CharField for experimenting for all field options.
Python3
from django.db import modelsfrom django.db.models import Model# Create your models here.class GeeksModel(Model): Lazyroar_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 break 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.
Now let us modify this error message to “The Geeks Field you entered is not unique.” using error_messages. Change the models.py to
Python3
from django.db import modelsfrom django.db.models import Model# Create your models here.class GeeksModel(Model): Lazyroar_field = models.CharField( max_length = 200, unique = True, error_messages ={ "unique":"The Geeks Field you entered is not unique." } ) |
Since models.py is modified run makemigrations and migrate again on the project. Open admin interface and try to make instance using the string “a” again.
You can see the modified error message. Therefore, error_messages modifies the field error messages. you can modify using other attributes such as null, blank, etc.
More Built-in Field Validations
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| Field Options | Description |
|---|---|
| Null | If True, Django will store empty values as NULL in the database. Default is False. |
| Blank | If True, the field is allowed to be blank. Default is False. |
| db_column | The name of the database column to use for this field. If this isn’t given, Django will use the field’s name. |
| Default | The 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_text | Extra “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_key | If True, this field is the primary key for the model. |
| editable | If 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_messages | The 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_text | Extra “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_name | A 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. |
| validators | A list of validators to run for this field. See the validators documentation for more information. |
| Unique | If True, this field must be unique throughout the table. |
