[FIXED] Assertion error at: Django-rest-Framework

Issue

I am using python 3.4, Django 1.7.1 (the version considered in the book), Postgres 9.3 and my IDE is Eclipse.

I have been studying the book "Lightweight Django - Elman and Lavin" and I have been stuck for days in the chapters 4 and 5, where we are supposed to use the rest framework and backbone.js. See for instance,

Lightweight Django - Chapters 4 and 5

Some days ago, I tried to code by myseld as presented in the book and also checking with the examples presented in the link above. However, since I was not going ahead, I decided to copy the code presented in the link above and tried to run. The same error has arisen:

AssertionError at /

Relational field must provide a `queryset` argument, or set read_only=`True`.

Request Method:     GET
Request URL:    http://127.0.0.1:8000/
Django Version:     1.7.1
Exception Type:     AssertionError
Exception Value: 

Relational field must provide a queryset argument, or set read_only=True.

Exception Location:     /usr/local/lib/python3.4/dist-packages/rest_framework/relations.py in __init__, line 35
Python Executable:  /usr/bin/python3
Python Version:     3.4.0
Python Path:    

['/home/daniel/workspace/Scrum',
 '/usr/lib/python3.4',
 '/usr/lib/python3.4/plat-i386-linux-gnu',
 '/usr/lib/python3.4/lib-dynload',
 '/usr/local/lib/python3.4/dist-packages',
 '/usr/lib/python3/dist-packages']

This error arises inside "relations.py, which belongs to the django-rest-framework. Since I am using the exact code presented in the link above, it is supposed to not have errors. Actually, the only piece of code that I changed was in the settings.py (after the error repeatedly happened):

Before:

DATABASES = {
    'default': {
        'ENGINE': 'django.db.backends.postgresql_psycopg2',
        'NAME': 'scrum',
    }
}

Now:

DATABASES = {
    'default': {
        'ENGINE': 'django.db.backends.postgresql_psycopg2',
        'NAME': 'scrum',
        'USER': 'daniel', 
        'PASSWORD': '12345',
        'HOST': '127.0.0.1',
        'PORT': '5432',        
    }

As you can see below, my user "daniel" has the following attributes:

Role name |                   Attributes                   | Member of | Description 
-----------+------------------------------------------------+-----------+-------------
 daniel    | Superuser, Create DB                           | {}        | 
 postgres  | Superuser, Create role, Create DB, Replication | {}        | 

Finally, it seems that I do not have any problem with the installation of psycopg2, since I was able to create "scrum" as presented below. In fact, the list of databases of my system is

                                      List of databases
   Name    |  Owner   | Encoding |   Collate   |    Ctype    |   Access privileges   
-----------+----------+----------+-------------+-------------+-----------------------
 postgres  | postgres | UTF8     | en_US.UTF-8 | en_US.UTF-8 | 
 scrum     | daniel   | UTF8     | en_US.UTF-8 | en_US.UTF-8 | =Tc/daniel           +
           |          |          |             |             | daniel=CTc/daniel
 template0 | postgres | UTF8     | en_US.UTF-8 | en_US.UTF-8 | =c/postgres          +
           |          |          |             |             | postgres=CTc/postgres
 template1 | postgres | UTF8     | en_US.UTF-8 | en_US.UTF-8 | =c/postgres          +
           |          |          |             |             | postgres=CTc/postgres

Can someone help me to discover the problem?

Solution

Read the DRF docs here.

In version 2.x a serializer class could sometimes automatically determine the queryset argument if a ModelSerializer class was being used.

This behavior is now replaced with always using an explicit queryset argument for writable relational fields.

You are just using a newer version of DRF than the authors of the code used, so you'll need to either use a lower version or fix the code.

In serializers.py there's this line:

assigned = serializers.SlugRelatedField(slug_field=User.USERNAME_FIELD, required=False)

You need to either add read_only=True or queryset=User.objects.all()

Answered By - Aldarund