karrot Backend Introduction

This is a beginner guide to karrot-backend by @id-gue and @mddemarie written for people who want to contribute to karrot, but aren't (yet) experienced Python/Django devs. Welcome and have fun!

Repository Structure

There are two separated Repos for Frontend and Backend.

Karrot-Frontend in JavaScript https://github.com/yunity/karrot-frontend

You don't need to do the setup for the frontend, but it might be useful to try out your backend through the frontend.

Karrot-Backend in Python Django REST https://github.com/yunity/karrot-backend

  • Python – object-oriented programming language
  • Django – Python framework for backend development
       Tutorial: https://docs.djangoproject.com/en/1.11/intro/tutorial01
  • Django REST Framework on top of django for building Web APIs
       Tutorial: http://www.django-rest-framework.org/tutorial/1-serialization

Both repositories are not directly connected – the data exchange works via an API.

01 Setup

We use Docker for the setup. How to build a Docker container is described in the README.md in the karrot-backend repository.

We would suggest to use 3 tabs in the shell:

  1. Tab for Communicating with git / GitHub (doing that inside the docker container might raise errors)

  2. Tab with Docker running for run manage.py commands

    Find out the name of your Docker Container: docker ps (examples: young_curie or amazing_lovelace)

    Run Docker with: docker exec -it <container_name> bash (After starting Docker your lines in the shell start with: (env))

    Running tests:

    python manage.py test (Please run the tests after your setup and every time you make a change in code.)

    After changing a model you have to migrate them:

    python manage.py makemigrations python manage.py migrate

    Leave Docker: exit

  3. Tab with Docker running to check what your server is doing

    Show the last 12 lines of the server output: docker logs -f <container_name> --tail "12"

    Note: The first line shown is an email address. Store it – we will need it for Swagger.

02 Project Architecture

Relationships in Backend

First of all, you have to have a Group Model, allowing to create objects like "Foodsavers Berlin". One Group usually has many Stores, like "Bakery Smith". Each store can define events where foodsavers can come by and save food. These events are called PickupDate (one time event) or PickupDateSeries (repetitive event).

core elements of foodsaving backend

As logged-in user, you can create and join a Group, what makes you a member. Afterwards, you can join or create a PickupDate event which takes place in the future, what makes you a collector.

Further actions are for example:

  • for member in Group: create/modify/join/leave
  • for member in Store: create/update/delete
  • for collector in PickupDate/PickupDateSeries: create/join/update/delete

Collectors have also an option after food pickup to leave feedback.

Foodsaving Apps

At the moment (September 2017) there are 15 Apps (= folders) in foodsaving. Not all of them are in use or critical for karrot.world since the project is under development and the dev team tries different approaches.

Important apps are for example:

  • groups (see above)
  • users (user data and user profile, reset user password, change password etc.)
  • userauth (login and logout)
  • base (most models in the code inherit from the models created there)
  • tests (the test coverage is very high - some of the tests are in the test app – others in the other apps)
  • stores and history might need a bit more explanation:


In models.py in stores, you can find classes for Stores and Feedback, as well as PickupDate and PickupDateSeries. The last two refer to pickup-date and pickup-series in Swagger (see chapter "Server and Swagger") and contain appropriate data fields. PickupDateManager with the method process_finished_pickup_dates is an interesting class because it processes old pickups and moves them into history (even empty ones) - as a result you find PICKUP_MISSED or PICKUP_DONE in the database.


In history you find any action regarding stores, groups or pickup-dates/pickup-series from the past. As a result, you find here different HistoryTypus (just “typus” in database), e.g. PICKUP_JOIN and additional data about that action. This helps to keep a track of all actions.

03 Stores app in detail

We want to dig a bit deeper into the app Stores (a) to give you an example of how the foodsaving apps work and (b) because there is too much functionality inside that you might like to know. If you haven't already opened the code in your editor: do it now! Open the stores app and have a look at the files:

  1. models.py Here you define which database tables you want to have and what the fields/columns should store in the database. One model (or class) defines one database table. Let's have a look at the model Feedback which creates four database fields (and two fields for the id and a time stamp, but these are created automatically here). The following line creates a field with the name comment.

    comment = models.CharField(max_length=settings.DESCRIPTION_MAX_LENGTH, blank=True)

    The type CharField says that comment will be stored as string in database. The maximum string length is given as DESCRIPTION_MAX_LENGTH in the file settings.py. The entry can be saved even if the comment field is blank.

  1. serializers.py The models we created in models.py are python objects. But these are not very useful in order to access the API – so we convert them to JSON objects with serializers. Our Feedback model has a FeedbackSerialzer which inherits many functions from ModelSerializers. But there are also new functions like validate_about. (user is a member of group, that member joined the pickup and the pickup is in the future). This validator checks if a user is allowed to give feedback about a certain pickup. (Validation within a Serializer might sound strange, but it's common in the REST framework. See Validators in the documentation)

  2. permissions.py Another possibility to check if something is allowed are permissions. They are used in api.py. Here is for example the permission IsNotFull that permits a member to join the pickup event only if it is not full.

  3. api.py The api defines how the data stored in the database can be accessed via API. The used HTTP methods (like GET, POST or PATCH) are described in chapter 03 Server and Swagger.

    Instead of normal Views we use whole ViewSets which allow to combine the logic for a set of related views. Have a look on the class FeedbackViewSet. You will notice that most HTTP methods (like GET) are not defined there but in an imported mixin. Each mixin contains whole logic for creating a single HTTP request. The ViewSets are connected with urls.py and defined there in form of a url.

  4. factories.py In a Factory you can create sample data used in the tests.

  5. A folder with tests The test coverage of the project is very good and Circle CI will answer in angry red if you try to push untested or non-functioning code.

    Have a look on the class FeedbackTest in test_feedback_api.py. First we create all data we need in the setUpClass we are going to use in our tests. Then we test step by step if the expected result is assertEqual to the actual result. (The chapter '01 Setup' explains how to run the tests in the shell.)

  6. A folder with migrations: You don't have to care about them a lot here. They are generated automatically when you run python manage.py makemigrations in the shell with Docker active.

Please also have a look on the used urls in config/urls.py and on the archive functions in foodsaving/history.

04 Server and Swagger

Why do you need the server output in the shell?

On one hand, this way you will notice when an error occurred (or worse, when the server crashed). On the other hand, you can observe the communication with the server while you interact with it.

Furthermore, you see an automatically generated mail address when you start running your docker container (see chapter Setup). Use this and the password 123 to login to Swagger in your browser:

Why Swagger?

Swagger shows you the API endpoints that are defined in the api.py files in the apps groups, stores etc. One of the API endpoint is pickup-dates.

You can use HTTP methods like:

  • GET: query data from database
  • POST: submits new entry into database
  • PATCH: modifies one entry in database based on given id
  • DELETE: deletes one entry from database based on gived id

You can also add additional functionalities to your API endpoint like:

  • GET /api/..../{id}/: displays one entry from database based on given (e.g. pickup-date) id
  • POST /api/..../{join}/: the user/member joins the group/store/pickup
  • POST /api/..../{leave}/: the user/member can leave the group/store/pickup
  • any other functionality added to GET, POST, PATCH or DELETE

The Database is automatically populated with sample data if you use Docker. But there are missing connections between: being user -> being member -> being collector -> pick up the food. You can create these connections in Swagger for testing purposes.

TIP: If you want, you can populate the database writing some querysets in the Django Shell and then look it up in Swagger. Or you can open PostgreSQL and populate the database there.

Response-request Cycle

Whenever you paste the url into your browser and hit Enter, you send a request to your local server sitting on your computer (live web sites have their own host server and domain). It depends if you want to GET data or POST data. The server will use the given URL, execute some functionality on the server (probably this includes accessing the database) and respond with a view that you can see in your browser.

05 Tests

Every time you run the tests (like described in the chapter Setup), an additional test database gets created. After the tests are done, it gets deleted. It is not connected to the database you use in Swagger. Therefore, we need to populate it for testing new functionality.

The common structure of the tests is:

  • Every class in the models.py, api.py and filters.py should have a corresponding class for testing (e.g. the class FeedbackViewSet in the file api.py gets tested in the class FeedbackTest in the file test–feedback–api.py)
  • The test class begins with a setUpClass. Here the database gets populated. Therefore, you can use: a) a Factory (like the member in FeedbackTest which gets created in the UserFactory) or b) you create the needed objects directly with querysets
  • Every test case should have its own function below SetUpClass to make bug fixing in the future easier

In the project, there are 2 types of tests:

1. Integration tests - test not only one class/function in a file but whole functionality of one part of the project. The server is taken as a 'black box', that is, we do not check what it actually does internally. We only verify that it creates an HTTP response that matches our expectations.

2. Unit tests - test only one function/class, e.g. the model Feedback is tested in test-models.py.

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