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394zj5">.css-394zj5{background:none;display:inline-block;margin:3px 5px;padding:2px 10px;font-weight:bold;color:#000000;background-color:#F2F2F2;}</style><a active="false" class="css-394zj5" href="/category/django">Django</a></div></div></div></div><style data-emotion="css rv9vlo">.css-rv9vlo{margin:auto;max-width:1024px;background:#FFFFFF;padding:2rem;font-size:1.07rem;line-height:1.7rem;margin-top:-80px;word-break:break-word;}.css-rv9vlo img{max-width:100%;height:auto;}.css-rv9vlo blockquote{position:relative;padding-left:1em;border-left:0.2em solid #FD7500;font-style:italic;margin:0;color:#5E5E5E;}.css-rv9vlo h2{margin:2.5rem 0 1.3rem 0;line-height:2.6rem;cursor:pointer;}.css-rv9vlo p:first-child,.css-rv9vlo h2:first-child,.css-rv9vlo h3:first-child{margin-top:0;line-height:2rem;}.css-rv9vlo h3{margin:2rem 0 1rem 0;cursor:pointer;}.css-rv9vlo blockquote code,.css-rv9vlo p code,.css-rv9vlo h1 code,.css-rv9vlo h2 code,.css-rv9vlo h3 code,.css-rv9vlo h4 code,.css-rv9vlo h5 code,.css-rv9vlo ol code,.css-rv9vlo ul code{padding:3px 8px;font-size:0.8em;background:#DEDEDE;border-radius:3px;}.css-rv9vlo blockquote code{font-style:normal;color:#000000;}.css-rv9vlo figure[class~='kg-embed-card']{position:relative;overflow:hidden;width:100%;padding-top:56.25%;margin:0;}.css-rv9vlo figure[class~='kg-embed-card'] iframe{top:0;left:0;bottom:0;right:0;width:100%;height:100%;}.css-rv9vlo figure[class~='kg-image-card']{margin:0;}.css-rv9vlo table{border:1px solid #E7E7E76E;border-collapse:collapse;display:block;max-width:-moz-fit-content;max-width:-webkit-fit-content;max-width:-moz-fit-content;max-width:fit-content;overflow-x:auto;white-space:nowrap;margin:10px 0;}.css-rv9vlo table th,.css-rv9vlo table td{border:1px solid #E7E7E76E;padding:10px;}.css-rv9vlo table tr:nth-of-type(even){background-color:#F2F2F2;}.css-rv9vlo table tr:hover{background-color:#E7E7E76E;}.css-rv9vlo table th{padding:12px;text-align:left;background-color:#929292;color:#FFFFFF;}@media only screen and (max-width: 768px){.css-rv9vlo{padding:0.9rem;margin-top:-50px;}}.css-rv9vlo pre{overflow-y:auto;max-width:calc(100vw - 40px);}</style><div class="blogPostContainer css-rv9vlo"><blockquote>This blog post was inspired by the following <a href="https://github.com/HackSoftware/Django-Styleguide/pull/119">pull request</a> towards our <a href="https://github.com/HackSoftware/Django-Styleguide">Django Styleguide</a></blockquote><p>When it comes to using <code>auto_now</code> and <code>auto_now_add</code> for the usual <code>created_at</code> / <code>updated_at</code> timestamps - I've always looked at the Django documentation, to figure out the exact behavior.</p><p>And the thing that was always lacking was an example. <strong>So this article should serve as an example!</strong></p><p><strong>But first, lets start with the problem we want to solve.</strong></p><p>Lets say we have the following <code>BaseModel</code> definition:</p><pre><code class="language-python">class BaseModel(models.Model): created_at = models.DateTimeField() updated_at = models.DateTimeField() class Meta: abstract = True</code></pre><p>Now, we might want to leverage <code>auto_now</code>, <code>auto_now_add</code> and/or <code>default</code>, in order for Django to take care of the values for us.</p><p>At the time of writing, in our <a href="https://github.com/HackSoftware/Django-Styleguide-Example">Styleguide Example Project</a>, the definition of the <code>BaseModel</code> is as follows (ignoring indexes, since they are not the focus of the article):</p><pre><code class="language-python">class BaseModel(models.Model): created_at = models.DateTimeField(default=timezone.now) updated_at = models.DateTimeField(auto_now=True) class Meta: abstract = True</code></pre><p>Which looks strange - we have <code>default</code> for <code>created_at</code>, but <code>auto_now</code> for <code>updated_at</code> 🤔</p><p>In order to decide what we actually need, we are going to explore how the different options behave:</p><ol><li>Using <code>auto_now_add</code> and <code>auto_now</code>.</li><li>Mixing <code>default</code> and <code>auto_now</code>.</li><li>Using only <code>default</code></li></ol><p>The approach we are going to take is:</p><ol><li><strong>Have a simple model for each of the options.</strong></li><li><strong>Assert the behavior writing tests.</strong></li></ol><p>Lets start with the models:</p><pre><code class="language-python">from django.db import models from django.utils import timezone class TimestampsWithAuto(models.Model): created_at = models.DateTimeField(auto_now_add=True) updated_at = models.DateTimeField(auto_now=True) class TimestampsWithAutoAndDefault(models.Model): created_at = models.DateTimeField(default=timezone.now) updated_at = models.DateTimeField(auto_now=True) class TimestampsWithDefault(models.Model): created_at = models.DateTimeField(default=timezone.now) updated_at = models.DateTimeField(default=timezone.now) </code></pre><h2 id="using-autonowadd-and-autonow">Using <code>auto_now_add</code> and <code>auto_now</code></h2><p>Perhaps, this is the case you can find most often on the internet &amp; thus - <strong>the one that's most often copy-pasted in projects.</strong></p><pre><code class="language-python">class TimestampsWithAuto(models.Model): created_at = models.DateTimeField(auto_now_add=True) updated_at = models.DateTimeField(auto_now=True)</code></pre><p>What's the behavior here?</p><p>Reading from <a href="https://docs.djangoproject.com/en/4.1/ref/models/fields/#django.db.models.DateField">Django's documentation</a>, we have the following:</p><p>For <code>auto_now</code>:</p><blockquote>Automatically set the field to now every time the object is saved. Useful for “last-modified” timestamps. Note that the current date is <em>always</em> used; it’s not just a default value that you can override.</blockquote><p>For <code>auto_now_add</code>:</p><blockquote>Automatically set the field to now when the object is first created. Useful for creation of timestamps. Note that the current date is <em>always</em> used; it’s not just a default value that you can override. So even if you set a value for this field when creating the object, it will be ignored.</blockquote><p>So if we use both <code>auto_now</code> and <code>auto_now_add</code>, we'll get the following behavior:</p><ol><li><code>created_at</code> is going to be populated with <code>timezone.now()</code> upon creation. A<em>s a reference, check this -</em> <a href="https://github.com/django/django/blob/main/django/db/models/fields/__init__.py#L1573">https://github.com/django/django/blob/main/django/db/models/fields/__init__.py#L1573</a></li><li><code>updated_at</code> is going to be populated with <code>timezone.now()</code> whenever the object is saved via <code>.save()</code>.</li><li>Even if you explicitly pass values to either <code>created_at</code> or <code>updated_at</code>, <strong>those values are going to be ignored!</strong></li></ol><p><strong>The 3rd point is extremely important.</strong> This means - if we use <code>auto_now</code> and <code>auto_now_add</code>, <strong>we won't have control over those values.</strong></p><p>Lets illustrate this behavior with a simple test against the model <em>(for more on what to test in a Django project, you can watch this talk from DjangoCon Europe 2022 - <a href="https://youtu.be/PChaEAIsQls">https://youtu.be/PChaEAIsQls</a>)</em></p><pre><code class="language-python">from datetime import timedelta from django.test import TestCase from django.utils import timezone from styleguide_example.common.models import ( TimestampsWithAuto, class TimestampsTests(TestCase): def test_timestamps_with_auto_behavior(self): Timestamps are set automatically obj = TimestampsWithAuto() obj.full_clean() obj.save() self.assertIsNotNone(obj.created_at) self.assertIsNotNone(obj.updated_at) self.assertNotEqual(obj.created_at, obj.updated_at) Timestamps cannot be overridden timestamp = timezone.now() - timedelta(days=1) obj = TimestampsWithAuto(created_at=timestamp, updated_at=timestamp) obj.full_clean() obj.save() self.assertNotEqual(timestamp, obj.created_at) self.assertNotEqual(timestamp, obj.updated_at) updated_at gets auto updated, while created_at stays the same obj = TimestampsWithAuto() obj.full_clean() obj.save() original_created_at = obj.created_at original_updated_at = obj.updated_at obj.save() # Get a fresh object obj = TimestampsWithAuto.objects.get(id=obj.id) self.assertEqual(original_created_at, obj.created_at) self.assertNotEqual(original_updated_at, obj.updated_at)</code></pre><p>The tests are passing, confirming the described behavior above.</p><h2 id="mixing-default-and-autonow">Mixing <code>default</code> and <code>auto_now</code></h2><p>Lets look at the next example:</p><pre><code class="language-python">class TimestampsWithAutoAndDefault(models.Model): created_at = models.DateTimeField(default=timezone.now) updated_at = models.DateTimeField(auto_now=True) </code></pre><p>At first, this looks a bit strange, since it relies on <code>default</code> for <code>created_at</code>.</p><p>Here's the behavior of <code>default</code>, according to <a href="https://docs.djangoproject.com/en/4.1/ref/models/fields/#default">Django's documentation</a>:</p><blockquote>The default value is used when new model instances are created and a value isn’t provided for the field. When the field is a primary key, the default is also used when the field is set to <code><strong>None</strong></code>.</blockquote><p>Okay, looks like it's going to give us the same behavior as with <code>auto_now_add=True</code>, with one exception - <strong>we can override the value</strong> for <code>created_at</code>, if we want to.</p><p>So if we use both <code>default</code> and <code>auto_now</code>, we'll get the following behavior:</p><ol><li><code>created_at</code> is going to be populated with <code>timezone.now()</code> upon creation.</li><li><code>updated_at</code> is going to be populated with <code>timezone.now()</code> whenever the object is saved via <code>.save()</code>.</li><li>We can pass a different value for <code>created_at</code>.</li><li>Even if we explicitly pass value to <code>updated_at</code>, <strong>it's going to be ignored.</strong></li></ol><p>Lets illustrate this with tests:</p><pre><code class="language-python">from datetime import timedelta from django.test import TestCase from django.utils import timezone from styleguide_example.common.models import ( TimestampsWithAutoAndDefault, class TimestampsTests(TestCase): def test_timestamps_with_mixed_behavior(self): Timestamps are set automatically / by default obj = TimestampsWithAutoAndDefault() obj.full_clean() obj.save() self.assertIsNotNone(obj.created_at) self.assertIsNotNone(obj.updated_at) self.assertNotEqual(obj.created_at, obj.updated_at) Some timestamps can be overridden timestamp = timezone.now() - timedelta(days=1) obj = TimestampsWithAutoAndDefault(created_at=timestamp, updated_at=timestamp) obj.full_clean() obj.save() # This is default self.assertEqual(timestamp, obj.created_at) # This is auto_now self.assertNotEqual(timestamp, obj.updated_at) updated_at gets auto updated, while created_at stays the same obj = TimestampsWithAutoAndDefault() obj.full_clean() obj.save() original_created_at = obj.created_at original_updated_at = obj.updated_at obj.save() # Get a fresh object obj = TimestampsWithAutoAndDefault.objects.get(id=obj.id) self.assertEqual(original_created_at, obj.created_at) self.assertNotEqual(original_updated_at, obj.updated_at)</code></pre><p>Again, this has a similar behavior as our first example, with the exception that we can now provide values for <code>created_at</code>!</p><h2 id="using-only-default">Using only <code>default</code></h2><p>Since we are here, lets explore the behavior of having both timestamps using <code>default</code>:</p><pre><code class="language-python">class TimestampsWithDefault(models.Model): created_at = models.DateTimeField(default=timezone.now) updated_at = models.DateTimeField(default=timezone.now)</code></pre><p>Following the <a href="https://docs.djangoproject.com/en/4.1/ref/models/fields/#default">documentation</a> for <code>default</code>, the behavior should be the following:</p><ol><li><code>created_at</code> is getting a <code>timezone.now()</code> value upon creation.</li><li><code>updated_at</code> is getting a <code>timezone.now()</code> value upon creation.</li><li>Unless the value is explicitly <code>None</code>, both <code>created_at</code> and <code>updated_at</code> <strong>are not going to be updated upon <code>.save()</code>.</strong></li><li>We have flexibility over the values for both <code>created_at</code> and <code>updated_at</code>, if we don't want to use the defaults.</li></ol><p>Lets illustrate that with tests:</p><pre><code class="language-python">from datetime import timedelta from django.test import TestCase from django.utils import timezone from styleguide_example.common.models import ( TimestampsWithDefault, class TimestampsTests(TestCase): def test_timestamps_with_default_behavior(self): Timestamps are set by default obj = TimestampsWithDefault() obj.full_clean() obj.save() self.assertIsNotNone(obj.created_at) self.assertIsNotNone(obj.updated_at) self.assertNotEqual(obj.created_at, obj.updated_at) Both timestamps can be overridden timestamp = timezone.now() - timedelta(days=1) obj = TimestampsWithDefault(created_at=timestamp, updated_at=timestamp) obj.full_clean() obj.save() self.assertEqual(timestamp, obj.created_at) self.assertEqual(timestamp, obj.updated_at) # And by transitivity self.assertEqual(obj.created_at, obj.updated_at) created_at / updated_at are not auto updated obj = TimestampsWithDefault() obj.full_clean() obj.save() original_created_at = obj.created_at original_updated_at = obj.updated_at obj.save() # Get a fresh object obj = TimestampsWithDefault.objects.get(id=obj.id) self.assertEqual(original_created_at, obj.created_at) self.assertEqual(original_updated_at, obj.updated_at)</code></pre><p>The tests confirm the described behavior above.</p><p>Since now we know the behavior of all 3 cases, we can make a better decision, based on our needs.</p><h2 id="the-opinionated-approach">The opinionated approach</h2><p>By the looks of it, all approaches from above can get the job done, but also, they might have some downsides.</p><p>For example, setting <code>updated_at</code> on new objects (with a value that's slightly different from <code>created_at</code> ) looks like a potential downside.</p><p><strong>Lets define our desired behavior and figure out how to achieve it:</strong></p><ol><li><code>created_at</code> should have a default, unless another value is provided (having the flexibility to change).</li><li><code>updated_at</code> should be empty (<code>null</code> / <code>None</code>) for new objects.</li><li><code>updated_at</code> should have a default value on updates, unless another value is provided (having the flexibility to change).</li></ol><p><strong>We can achieve the first 2 points with the following model definition:</strong></p><pre><code class="language-python">class TimestampsOpinionated(models.Model): created_at = models.DateTimeField(default=timezone.now) updated_at = models.DateTimeField(blank=True, null=True)</code></pre><p>Lets illustrate the behavior with tests:</p><pre><code class="language-python">from datetime import timedelta from django.test import TestCase from django.utils import timezone from styleguide_example.common.models import ( TimestampsOpinionated class TimestampsTests(TestCase): def test_timestamps_with_opinionated_behavior(self): created_at is only set by default obj = TimestampsOpinionated() obj.full_clean() obj.save() self.assertIsNotNone(obj.created_at) self.assertIsNone(obj.updated_at) Both timestamps can be overridden timestamp = timezone.now() - timedelta(days=1) obj = TimestampsOpinionated(created_at=timestamp, updated_at=timestamp) obj.full_clean() obj.save() self.assertEqual(timestamp, obj.created_at) self.assertEqual(timestamp, obj.updated_at) # And by transitivity self.assertEqual(obj.created_at, obj.updated_at) updated_at is not auto updated, created_at stays the same obj = TimestampsOpinionated() obj.full_clean() obj.save() original_created_at = obj.created_at original_updated_at = obj.updated_at