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autoupdated database fields for model methods

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django-computedfields

django-computedfields provides autoupdated database fields for model methods.

Tested with Django 2.2 and 3.0 (Python 3.6 to 3.8).

Example

Just derive your model from ComputedFieldsModel and place the @computed decorator at a method:

from django.db import models
from computedfields.models import ComputedFieldsModel, computed

class MyModel(ComputedFieldsModel):
    name = models.CharField(max_length=32)

    @computed(models.CharField(max_length=32), depends=[['self', ['name']]])
    def computed_field(self):
        return self.name.upper()

computed_field will be turned into a real database field and can be accessed and searched like any other database field. During saving the associated method gets called and it’s result written to the database. With the method compute('fieldname') you can inspect the value that will be written, which is useful if you have pending changes:

>>> person = MyModel(forename='berty')
>>> person.computed_field             # empty since not saved yet
>>> person.compute('computed_field')  # outputs 'BERTY'
>>> person.save()
>>> person.computed_field             # outputs 'BERTY'

The depends keyword argument can be used with any relation to indicate dependencies to fields on other models as well:

from django.db import models
from computedfields.models import ComputedFieldsModel, computed

class MyModel(ComputedFieldsModel):
    name = models.CharField(max_length=32)
    fk = models.ForeignKey(SomeModel)

    @computed(models.CharField(max_length=32), depends=[['self', ['name']], ['fk', ['fieldname']]])
    def computed_field(self):
        return self.name.upper() + self.fk.fieldname

Now changes to self.name or fk.fieldname will update computed_field.

Documentation

The documentation can be found here.

Changelog

  • 0.1.3
    • better multi table inheritance support and test cases
    • explicit docs for multi table inheritance
  • 0.1.2
    • bugfix: o2o reverse name access
    • add docs about model inheritance support
  • 0.1.1
    • bugfix: add missing migration
  • 0.1.0
    • fix recursion on empty queryset
    • dependency expansion on M2M fields
    • m2m_changed handler with filtering on m2m fields
    • remove custom metaclass, introducing Resolver class
    • new decorator @precomputed for custom save methods
    • old depends syntax removed
    • docs update
  • 0.0.23:
    • Bugfix: Fixing leaking computed fields in model inheritance.
  • 0.0.22:
    • Automatic dependency expansion on reverse relations.
    • Example documentation.
  • 0.0.21:
    • Bugfix: Fixing undefined _batchsize for pickled map usage.
  • 0.0.20
    • Use bulk_update for computed field updates.
    • Allow custom update optimizations with select_related and prefetch_related.
    • Respect computed field MRO in compute.
    • Allow updates on local computed fields from update_dependent simplifying bulk actions on ComputedFieldsModel.
  • 0.0.19
    • Better graph expansion on relation paths with support for update_fields.
  • 0.0.18
    • New depends syntax deprecating the old one.
    • MRO of local computed field methods implemented.
  • 0.0.17
    • Dropped Python 2.7 and Django 1.11 support.

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