# Extending Models

Below is a list of tasks to consider when adding a new field to a core model.

## 1. Add the field to the model class

Add the field to the model, taking care to address any of the following conditions.

* When adding a GenericForeignKey field, you may need add an index under `Meta` for its two concrete fields. (This is required only for non-unique GFK relationships, as the unique constraint introduces its own index.) For example:

    ```python
    class Meta:
        indexes = (
            models.Index(fields=('object_type', 'object_id')),
        )
    ```

## 2. Generate and run database migrations

[Django migrations](https://docs.djangoproject.com/en/stable/topics/migrations/) are used to express changes to the database schema. In most cases, Django can generate these automatically, however very complex changes may require manual intervention. Always remember to specify a short but descriptive name when generating a new migration.

```
./manage.py makemigrations <app> -n <name> --no-header
./manage.py migrate
```

Where possible, try to merge related changes into a single migration. For example, if three new fields are being added to different models within an app, these can be expressed in a single migration. You can merge a newly generated migration with an existing one by combining their `operations` lists.

!!! warning "Do not alter existing migrations"
    Migrations can only be merged within a release. Once a new release has been published, its migrations cannot be altered (other than for the purpose of correcting a bug).

## 3. Add validation logic to `clean()`

If the new field introduces additional validation requirements (beyond what's included with the field itself), implement them in the model's `clean()` method. Remember to call the model's original method using `super()` before or after your custom validation as appropriate:

```
class Foo(models.Model):

    def clean(self):
        super().clean()

        # Custom validation goes here
        if self.bar is None:
            raise ValidationError()
```

## 4. Update relevant querysets

If you're adding a relational field (e.g. `ForeignKey`) and intend to include the data when retrieving a list of objects, be sure to include the field using `prefetch_related()` as appropriate. This will optimize the view and avoid extraneous database queries.

## 5. Update API serializer

Extend the model's API serializer in `<app>.api.serializers` to include the new field.

## 6. Add fields to forms

Extend any forms to include the new field(s) as appropriate. These are found under the `forms/` directory within each app. Common forms include:

* **Credit/edit** - Manipulating a single object
* **Bulk edit** - Performing a change on many objects at once
* **CSV import** - The form used when bulk importing objects in CSV format
* **Filter** - Displays the options available for filtering a list of objects (both UI and API)

## 7. Extend object filter set

If the new field should be filterable, add it to the `FilterSet` for the model. If the field should be searchable, remember to query it in the FilterSet's `search()` method.

## 8. Add column to object table

If the new field will be included in the object list view, add a column to the model's table. For simple fields, adding the field name to `Meta.fields` will be sufficient. More complex fields may require declaring a custom column. Also add the field name to `default_columns` if the column should be present in the table by default.

## 9. Update the SearchIndex

Where applicable, add the new field to the model's SearchIndex for inclusion in global search.

## 10. Update the UI templates

Edit the object's view template to display the new field. There may also be a custom add/edit form template that needs to be updated.

## 11. Create/extend test cases

Create or extend the relevant test cases to verify that the new field and any accompanying validation logic perform as expected. This is especially important for relational fields. NetBox incorporates various test suites, including:

* API serializer/view tests
* Filter tests
* Form tests
* Model tests
* View tests

Be diligent to ensure all the relevant test suites are adapted or extended as necessary to test any new functionality.

## 12. Update the model's documentation

Each model has a dedicated page in the documentation, at `models/<app>/<model>.md`. Update this file to include any relevant information about the new field.
