Django rest framework: serializers.ReadOnlyField() doesn't display field in browsable api

Question

Django rest framework: How can I display a read-only field in the browsable api?

When I add result = serializers.CharField(read_only=True) to my model serializer, the form no longer renders the results field.

I understand the security concerns of a user removing the disabled attribute on a form input (though I am surprised django doesn't handle this natively), so how can I implement a read-only field in the api.html template from result?

serializers.py

class SnippetSerializer(serializers.HyperlinkedModelSerializer):
owner = serializers.ReadOnlyField(source='owner.username')
result = serializers.CharField(read_only=True)

class Meta:
    model = Snippet
    fields = ('title', 'code', 'owner', 'url', 'result')

I am new to the django-rest framework, so any help would be appreciated!

Answer 1

You have 2 options:

1. either to calculate the result in the model

2. or to add the field within the serialization

What you choose depends on whether you want to use that calculated result also somewhere else and whether you can touch models.

When you want to calculate the result in the model

Follow the example of Django's derived fullname, somewhere around: https://github.com/django/django/blob/master/django/contrib/auth/models.py#L348

Or explained here in the doc: https://docs.djangoproject.com/en/dev/topics/db/models/#model-methods

That will act as a read only field for DRF automatically.

You can see the usage in the code bellow (get_full_name).

When you want to add field within serialization

You have the answer in the DRF docs: http://www.django-rest-framework.org/api-guide/fields/#serializermethodfield

SerializerMethodField This is a read-only field...It can be used to add any sort of data to the serialized representation of your object.

Example hours_since_joined in serializers.py:

from django.contrib.auth.models import User, Group
from rest_framework import serializers
from django.utils.timezone import now

class UserSerializer(serializers.HyperlinkedModelSerializer):
    hours_since_joined = serializers.SerializerMethodField()
    class Meta:
        model = User
        fields = ('url', 'username', 'email', 'groups', 'hours_since_joined', 'first_name', 'last_name', 'get_full_name' )

    def get_hours_since_joined(self, obj):
        return (now() - obj.date_joined).total_seconds() // 3600

class GroupSerializer(serializers.HyperlinkedModelSerializer):
    class Meta:
        model = Group
        fields = ('url', 'name', 'user_set')

For your case:

class SnippetSerializer(serializers.HyperlinkedModelSerializer):
    owner = serializers.ReadOnlyField(source='owner.username')
    result = serializers.SerializerMethodField()

    class Meta:
        model = Snippet
        fields = ('title', 'code', 'owner', 'url', 'result')

    def get_result(self, obj):
        # code here to calculate the result
        # or return obj.calc_result() if you have that calculation in the model
        return "some result"

To show the added fields in the DRF's browsable API

You need to list them in the Meta's fields - see example above. That will present that in the browsable output on requests. However it will not show them in the DRF's HTML form. The reason is that the HTML form is used for submitting information only, so the restframework template skips the read only field in rendering.

As you can see the full name and hours since joined is not rendered in the form, but is available for API:

If you want to show read only fields also on the form

you need to override the restframework templates.

• make sure your templates are loaded before the restframework's (i.e. your app is above the restframework in the settings.py)
• use the templates dir under your app
• create subdir in your templates dir: restframework/horizontal

• copy the form.html and input.html from Python's Lib\site-packages\rest_framework\templates\rest_framework\horizontal\

• change the form.html

{% load rest_framework %}
{% for field in form %}
    {% render_field field style=style %}
{% endfor %}

• change input line in the input.html (adding disabled attribute)

  <input name="{{ field.name }}"  {% if field.read_only %}disabled{% endif %} {% if style.input_type != "file" %}class="form-control"{% endif %} type="{{ style.input_type }}" {% if style.placeholder %}placeholder="{{ style.placeholder }}"{% endif %} {% if field.value %}value="{{ field.value }}"{% endif %}>
  

The result: