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1386c379b7
* add test for reproducing FieldTracker update_fields issue FieldTracker does not see update_fields changed in tracked model.save() method * Patch Model.save_base instead of save This change allows proper capturing update_fields kwarg if is is changed in overridden save() method. * Add some details about FieldTracker implementation * Mention FieldTracker behavior change
343 lines
11 KiB
ReStructuredText
343 lines
11 KiB
ReStructuredText
=======================
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Miscellaneous Utilities
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=======================
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.. _Choices:
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Choices
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=======
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``Choices`` provides some conveniences for setting ``choices`` on a Django model field:
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.. code-block:: python
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from model_utils import Choices
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class Article(models.Model):
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STATUS = Choices('draft', 'published')
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status = models.CharField(choices=STATUS, default=STATUS.draft, max_length=20)
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A ``Choices`` object is initialized with any number of choices. In the
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simplest case, each choice is a string; that string will be used both
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as the database representation of the choice, and the human-readable
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representation. Note that you can access options as attributes on the
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``Choices`` object: ``STATUS.draft``.
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But you may want your human-readable versions translated, in which
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case you need to separate the human-readable version from the DB
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representation. In this case you can provide choices as two-tuples:
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.. code-block:: python
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from model_utils import Choices
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class Article(models.Model):
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STATUS = Choices(('draft', _('draft')), ('published', _('published')))
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status = models.CharField(choices=STATUS, default=STATUS.draft, max_length=20)
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But what if your database representation of choices is constrained in
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a way that would hinder readability of your code? For instance, you
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may need to use an ``IntegerField`` rather than a ``CharField``, or
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you may want the database to order the values in your field in some
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specific way. In this case, you can provide your choices as triples,
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where the first element is the database representation, the second is
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a valid Python identifier you will use in your code as a constant, and
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the third is the human-readable version:
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.. code-block:: python
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from model_utils import Choices
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class Article(models.Model):
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STATUS = Choices((0, 'draft', _('draft')), (1, 'published', _('published')))
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status = models.IntegerField(choices=STATUS, default=STATUS.draft)
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You can index into a ``Choices`` instance to translate a database
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representation to its display name:
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.. code-block:: python
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status_display = Article.STATUS[article.status]
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Option groups can also be used with ``Choices``; in that case each
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argument is a tuple consisting of the option group name and a list of
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options, where each option in the list is either a string, a two-tuple,
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or a triple as outlined above. For example:
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.. code-block:: python
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from model_utils import Choices
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class Article(models.Model):
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STATUS = Choices(('Visible', ['new', 'archived']), ('Invisible', ['draft', 'deleted']))
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Choices can be concatenated with the ``+`` operator, both to other Choices
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instances and other iterable objects that could be converted into Choices:
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.. code-block:: python
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from model_utils import Choices
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GENERIC_CHOICES = Choices((0, 'draft', _('draft')), (1, 'published', _('published')))
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class Article(models.Model):
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STATUS = GENERIC_CHOICES + [(2, 'featured', _('featured'))]
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status = models.IntegerField(choices=STATUS, default=STATUS.draft)
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Should you wish to provide a subset of choices for a field, for
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instance, you have a form class to set some model instance to a failed
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state, and only wish to show the user the failed outcomes from which to
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select, you can use the ``subset`` method:
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.. code-block:: python
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from model_utils import Choices
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OUTCOMES = Choices(
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(0, 'success', _('Successful')),
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(1, 'user_cancelled', _('Cancelled by the user')),
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(2, 'admin_cancelled', _('Cancelled by an admin')),
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)
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FAILED_OUTCOMES = OUTCOMES.subset('user_cancelled', 'admin_cancelled')
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The ``choices`` attribute on the model field can then be set to
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``FAILED_OUTCOMES``, thus allowing the subset to be defined in close
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proximity to the definition of all the choices, and reused elsewhere as
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required.
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Field Tracker
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=============
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A ``FieldTracker`` can be added to a model to track changes in model fields. A
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``FieldTracker`` allows querying for field changes since a model instance was
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last saved. An example of applying ``FieldTracker`` to a model:
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.. code-block:: python
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from django.db import models
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from model_utils import FieldTracker
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class Post(models.Model):
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title = models.CharField(max_length=100)
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body = models.TextField()
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tracker = FieldTracker()
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.. note::
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``django-model-utils`` 1.3.0 introduced the ``ModelTracker`` object for
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tracking changes to model field values. Unfortunately ``ModelTracker``
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suffered from some serious flaws in its handling of ``ForeignKey`` fields,
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potentially resulting in many extra database queries if a ``ForeignKey``
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field was tracked. In order to avoid breaking API backwards-compatibility,
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``ModelTracker`` retains the previous behavior but is deprecated, and
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``FieldTracker`` has been introduced to provide better ``ForeignKey``
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handling. All uses of ``ModelTracker`` should be replaced by
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``FieldTracker``.
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Summary of differences between ``ModelTracker`` and ``FieldTracker``:
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* The previous value returned for a tracked ``ForeignKey`` field will now
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be the raw ID rather than the full object (avoiding extra database
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queries). (GH-43)
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* The ``changed()`` method no longer returns the empty dictionary for all
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unsaved instances; rather, ``None`` is considered to be the initial value
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of all fields if the model has never been saved, thus ``changed()`` on an
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unsaved instance will return a dictionary containing all fields whose
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current value is not ``None``.
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* The ``has_changed()`` method no longer crashes after an object's first
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save. (GH-53).
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Accessing a field tracker
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-------------------------
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There are multiple methods available for checking for changes in model fields.
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previous
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~~~~~~~~
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Returns the value of the given field during the last save:
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.. code-block:: pycon
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>>> a = Post.objects.create(title='First Post')
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>>> a.title = 'Welcome'
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>>> a.tracker.previous('title')
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u'First Post'
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Returns ``None`` when the model instance isn't saved yet.
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If a field is `deferred`_, calling ``previous()`` will load the previous value from the database.
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.. _deferred: https://docs.djangoproject.com/en/2.0/ref/models/querysets/#defer
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has_changed
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~~~~~~~~~~~
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Returns ``True`` if the given field has changed since the last save. The ``has_changed`` method expects a single field:
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.. code-block:: pycon
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>>> a = Post.objects.create(title='First Post')
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>>> a.title = 'Welcome'
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>>> a.tracker.has_changed('title')
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True
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>>> a.tracker.has_changed('body')
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False
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The ``has_changed`` method relies on ``previous`` to determine whether a
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field's values has changed.
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If a field is `deferred`_ and has been assigned locally, calling ``has_changed()``
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will load the previous value from the database to perform the comparison.
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changed
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~~~~~~~
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Returns a dictionary of all fields that have been changed since the last save
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and the values of the fields during the last save:
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.. code-block:: pycon
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>>> a = Post.objects.create(title='First Post')
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>>> a.title = 'Welcome'
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>>> a.body = 'First post!'
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>>> a.tracker.changed()
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{'title': 'First Post', 'body': ''}
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The ``changed`` method relies on ``has_changed`` to determine which fields
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have changed.
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Tracking specific fields
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------------------------
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A fields parameter can be given to ``FieldTracker`` to limit tracking to
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specific fields:
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.. code-block:: python
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from django.db import models
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from model_utils import FieldTracker
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class Post(models.Model):
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title = models.CharField(max_length=100)
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body = models.TextField()
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title_tracker = FieldTracker(fields=['title'])
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An example using the model specified above:
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.. code-block:: pycon
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>>> a = Post.objects.create(title='First Post')
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>>> a.body = 'First post!'
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>>> a.title_tracker.changed()
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{'title': None}
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Tracking Foreign Key Fields
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---------------------------
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It should be noted that a generic FieldTracker tracks Foreign Keys by db_column name, rather than model field name, and would be accessed as follows:
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.. code-block:: python
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from django.db import models
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from model_utils import FieldTracker
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class Parent(models.Model):
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name = models.CharField(max_length=64)
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class Child(models.Model):
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name = models.CharField(max_length=64)
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parent = models.ForeignKey(Parent)
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tracker = FieldTracker()
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.. code-block:: pycon
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>>> p = Parent.objects.create(name='P')
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>>> c = Child.objects.create(name='C', parent=p)
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>>> c.tracker.has_changed('parent_id')
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To find the db_column names of your model (using the above example):
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.. code-block:: pycon
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>>> for field in Child._meta.fields:
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field.get_attname_column()
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('id', 'id')
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('name', 'name')
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('parent_id', 'parent_id')
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The model field name *may* be used when tracking with a specific tracker:
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.. code-block:: python
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specific_tracker = FieldTracker(fields=['parent'])
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But according to issue #195 this is not recommended for accessing Foreign Key Fields.
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Checking changes using signals
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------------------------------
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The field tracker methods may also be used in ``pre_save`` and ``post_save``
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signal handlers to identify field changes on model save.
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.. NOTE::
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Due to the implementation of ``FieldTracker``, ``post_save`` signal
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handlers relying on field tracker methods should only be registered after
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model creation.
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FieldTracker implementation details
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-----------------------------------
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.. code-block:: python
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from django.db import models
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from model_utils import FieldTracker, TimeStampedModel
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class MyModel(TimeStampedModel):
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name = models.CharField(max_length=64)
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tracker = FieldTracker()
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def save(self, *args, **kwargs):
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""" Automatically add "modified" to update_fields."""
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update_fields = kwargs.get('update_fields')
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if update_fields is not None:
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kwargs['update_fields'] = set(update_fields) | {'modified'}
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super().save(*args, **kwargs)
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# [...]
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instance = MyModel.objects.first()
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instance.name = 'new'
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instance.save(update_fields={'name'})
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This is how ``FieldTracker`` tracks field changes on ``instance.save`` call.
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1. In ``class_prepared`` handler ``FieldTracker`` patches ``save_base`` and
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``refresh_from_db`` methods to reset initial state for tracked fields.
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2. In ``post_init`` handler ``FieldTracker`` saves initial values for tracked
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fields.
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3. ``MyModel.save`` changes ``update_fields`` in order to store auto updated
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``modified`` timestamp. Complete list of saved fields is now known.
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4. ``Model.save`` does nothing interesting except calling ``save_base``.
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5. Decorated ``save_base()`` method calls ``super().save_base`` and all fields
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that have values different to initial are considered as changed.
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6. ``Model.save_base`` sends ``pre_save`` signal, saves instance to database and
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sends ``post_save`` signal. All ``pre_save/post_save`` receivers can query
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``instance.tracker`` for a set of changed fields etc.
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7. After ``Model.save_base`` return ``FieldTracker`` resets initial state for
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updated fields (if no ``update_fields`` passed - whole initial state is
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reset).
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8. ``instance.refresh_from_db()`` call causes initial state reset like for
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``save_base()``.
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