
Django Quantity Field
=====================

A Small django field extension allowing you to store quantities in certain units and perform conversions easily. Uses `pint <https://github.com/hgrecco/pint>`_ behind the scenes. Also contains a form field class and form widget that allows a user to choose alternative units to input data. The cleaned_data will output the value in the base_units defined for the field, eg: you specify you want to store a value in grams but will allow users to input either grams or ounces.

Help wanted
-----------

I am currently not working with Django anymore. Therefore the Maintenance of this project is not a priority for me anymore.
If there is anybody that could imagine helping out maintaining the project, send me a mail.

Compatibility
-------------

Requires django >= 5.2, and python 3.10/3.11/3.12/3.13/3.14

Tested with the following combinations:


* Django 5.2 (Python 3.10, 3.11, 3.12, 3.13, 3.14)
* Django 6.0 (Python 3.12, 3.13, 3.14)

Installation
------------

.. code-block::

   pip install django-pint



Simple Example
--------------

Best way to illustrate is with an example

.. code-block::

   # app/models.py

   from django.db import models
   from quantityfield.fields import QuantityField

   class HayBale(models.Model):
       weight = QuantityField('tonne')


Quantities are stored as float (Django FloatField) and retrieved like any other field

.. code-block::

   >> bale = HayBale.objects.create(weight=1.2)
   >> bale = HayBale.objects.first()
   >> bale.weight
   <Quantity(1.2, 'tonne')>
   >> bale.weight.magnitude
   1.2
   >> bale.weight.units
   'tonne'
   >> bale.weight.to('kilogram')
   <Quantity(1200, 'kilogram')>
   >> bale.weight.to('pound')
   <Quantity(2645.55, 'pound')>


If your base unit is atomic (i.e. can be represented by an integer), you may also use ``IntegerQuantityField`` and ``BigIntegerQuantityField``.

If you prefer exact units you can use the ``DecimalQuantityField``

You can also pass Quantity objects to be stored in models. These are automatically converted to the units defined for the field
( but can be converted to something else when retrieved of course ).

.. code-block::

   >> from quantityfield.units import ureg
   >> Quantity = ureg.Quantity
   >> pounds = Quantity(500 * ureg.pound)
   >> bale = HayBale.objects.create(weight=pounds)
   >> bale.weight
   <Quantity(0.226796, 'tonne')>


Use the inbuilt form field and widget to allow input of quantity values in different units

.. code-block::

   from quantityfield.fields import QuantityFormField

   class HayBaleForm(forms.Form):
       weight = QuantityFormField(base_units='gram', unit_choices=['gram', 'ounce', 'milligram'])


The form will render a float input and a select widget to choose the units.
Whenever cleaned_data is presented from the above form the weight field value will be a
Quantity with the units set to grams (values are converted from the units input by the user).
You also can add the ``unit_choices`` directly to the ``ModelField``. It will be propagated
correctly.

For comparative lookups, query values will be coerced into the correct units when comparing values,
this means that comparing 1 ounce to 1 tonne should yield the correct results.

.. code-block::

   less_than_a_tonne = HayBale.objects.filter(weight__lt=Quantity(2000 * ureg.pound))


You can also use a custom Pint unit registry in your project ``settings.py``

.. code-block::

   # project/settings.py

   from pint import UnitRegistry

   # django-pint will set the DJANGO_PINT_UNIT_REGISTER automatically
   # as application_registry
   DJANGO_PINT_UNIT_REGISTER = UnitRegistry('your_units.txt')
   DJANGO_PINT_UNIT_REGISTER.define('beer_bottle_weight = 0.8 * kg = beer')

   # app/models.py

   class HayBale(models.Model):
       # now you can use your custom units in your models
       custom_unit = QuantityField('beer')


Note: As the `documentation from pint <https://pint.readthedocs.io/en/latest/tutorial.html#using-pint-in-your-projects>`_
states quite clearly: For each project there should be only one unit registry.
Please note that if you change the unit registry for an already created project with
data in a database, you could invalidate your data! So be sure you know what you are
doing!
Still only adding units should be okay.

Development
-----------

Preparation
^^^^^^^^^^^

You need to install all Python Version that django-pint is compatible with.
In a *nix environment you best could use `pyenv <https://github.com/pyenv/pyenv>`_ to do so.

Furthermore, you need to install `tox <https://tox.wiki/en/latest/>`_ and `pre-commit <https://pre-commit.com/>`_ to lint and test.

You also need docker as our tests require a postgres database to run.
We don't use SQL lite as some bugs only occurred using a proper database.

I recommend using `pipx <https://pypa.github.io/pipx/>`_ to install them.


#. Install ``pipx`` (see pipx documentation), i.e. with ``python3 -m pip install --user pipx && python3 -m pipx ensurepath``
#. Install ``pre-commit`` running ``pipx install pre-commit``
#. Install ``tox``  running ``pipx install tox``
#. Install the ``tox-docker`` plugin ``pipx inject tox tox-docker``
#. Fork ``django-pint`` and clone your fork (see `Tutorial <https://docs.github.com/get-started/quickstart/contributing-to-projects>`_\ )
#. Change into the repo ``cd django-pint``
#. Activate ``pre-commit`` for the repo running ``pre-commit install``
#. Check that all linter run fine with the cloned version by running ``pre-commit run --all-files``
#. Check that all tests succeed by running ``tox``

**Congratulation** you successfully cloned and tested the upstream version of ``django-pint``.

Now you can work on your feature branch and test your changes using ``tox``.
Your code will be automatically linted and formatted by ``pre-commit`` if you commit your changes.
If it fails, simply add all changes and try again.
If this doesn't help look at the output of your ``git commit`` command.

Once you are done, `create a pull request <https://docs.github.com/pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/creating-a-pull-request-from-a-fork>`_.

Local development environment with Docker
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

To run a local development environment with Docker you need to run the following steps:
This is helpful if you have troubles installing ``postgresql`` or ``psycopg2-binary``.


#. ``git clone`` your fork
#. run ``cp .env.example .env``
#. edit ``.env`` file and change it with your credentials ( the postgres host should match the service name in docker-file so you can use "postgres" )
#. run ``cp tests/local.py.docker-example tests/local.py``
#. run ``docker-compose up`` in the root folder, this should build and start 2 containers, one for postgres and the other one python dependencies. Note you have to be in the `docker <https://stackoverflow.com/a/47078951/3813064>`_ group for this to work.
#. open a new terminal and run ``docker-compose exec app bash``\ , this should open a ssh console in the docker container
#. you can run ``pytest`` inside the container to see the result of the tests.

Updating the package
^^^^^^^^^^^^^^^^^^^^

`Python <https://endoflife.date/python>`_ and `Django <https://endoflife.date/django>`_ major versions have defined EOL.
To reduce the maintenance burden and encourage users to use version still receiving security updates any ``django-pint`` update should match all and only these version of Python and Django that are supported.
Updating these dependencies have to be done in multiple places:


* ``README.md``\ : Describing it to end users
* ``tox.ini``\ : For local testing
* ``pyproject.toml``\ : For usage with pip and displaying it in PyPi
* ``.github/workflows/test.yaml``\ : For the CI/CD Definition
