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@ -2,11 +2,10 @@
---
Django styleguide used in [HackSoft](https://hacksoft.io) projects.
Django styleguide that we use in [HackSoft](https://hacksoft.io).
Expect often updates as we discuss & decide upon different things.
If you want to check an existing project showing most of the styleguide, [check the Styleguide-Example](https://github.com/HackSoftware/Styleguide-Example)
1. We have a [`Styleguide-Example`](https://github.com/HackSoftware/Styleguide-Example) to show most of the styleguide in an actual project.
1. You can watch Radoslav Georgiev's [Django structure for scale and longevity](https://www.youtube.com/watch?v=yG3ZdxBb1oo) for the philosophy behind the styleguide.
**Table of contents:**
@ -15,7 +14,9 @@ If you want to check an existing project showing most of the styleguide, [check
- [Overview](#overview)
- [Cookie Cutter](#cookie-cutter)
- [Models](#models)
* [Custom validation](#custom-validation)
* [Base model](#base-model)
* [Validation - `clean` and `full_clean`](#validation---clean-and-full_clean)
* [Validation - constraints](#validation---constraints)
* [Properties](#properties)
* [Methods](#methods)
* [Testing](#testing)
@ -91,191 +92,274 @@ Few examples:
## Models
Models should take care of the data model and not much else.
### Base model
It's a good idea to define a `BaseModel`, that you can inherit.
Usually, fields like `created_at` and `updated_at` are perfect candidates to go into a `BaseModel`.
Defining a primary key can also go there. Potential candidate for that is the [`UUIDField`](https://docs.djangoproject.com/en/dev/ref/models/fields/#uuidf)
Here's an example `BaseModel`:
```python
from django.db import models
from django.utils import timezone
class BaseModel(models.Model):
created_at = models.DateTimeField(db_index=True, default=timezone.now)
updated_at = models.DateTimeField(auto_now=True)
class Meta:
abstract = True
```
Then, whenever you need a new model, just inherit `BaseModel`:
```python
class SomeModel(BaseModel):
pass
```
### Validation - `clean` and `full_clean`
Lets take a look at an example model:
```python
class Course(models.Model):
class Course(BaseModel):
name = models.CharField(unique=True, max_length=255)
start_date = models.DateField()
end_date = models.DateField()
attendable = models.BooleanField(default=True)
def clean(self):
if self.start_date >= self.end_date:
raise ValidationError("End date cannot be before start date")
```
students = models.ManyToManyField(
Student,
through='CourseAssignment',
through_fields=('course', 'student')
)
We are defining the model's `clean` method, because we want to make sure we get good data in our database.
teachers = models.ManyToManyField(
Teacher,
through='CourseAssignment',
through_fields=('course', 'teacher')
)
Now, in order for the `clean` method to be called, someone must call `full_clean` on an instance of our model, before saving.
slug_url = models.SlugField(unique=True)
**Our recommendation is to do that in the service, right before calling clean:**
repository = models.URLField(blank=True)
video_channel = models.URLField(blank=True)
facebook_group = models.URLField(blank=True)
```python
def course_create(*, name: str, start_date: date, end_date: date) -> Course:
obj = Course(name=name, start_date=start_date, end_date=end_date)
logo = models.ImageField(blank=True)
obj.full_clean()
obj.save()
public = models.BooleanField(default=True)
return obj
```
generate_certificates_delta = models.DurationField(default=timedelta(days=15))
This also plays well with Django admin, because the forms used there will trigger `full_clean` on the instance.
objects = CourseManager()
**We have few general rules of thumb for when to add validation in the model's `clean` method:**
1. If we are validating based on multiple, **non-relational fields**, of the model.
1. If the validation itself is simple enough.
**Validation should be moved to the service layer if:**
1. The validation logic is more complex.
1. Spanning relations & fetching additional data is required.
> It's OK to have validation both in `clean` and in the service, but we tend to move things in the service, if that's the case.
### Validation - constraints
As proposed in [this issue](https://github.com/HackSoftware/Django-Styleguide/issues/22), if you can do validation using [Django's constraints](https://docs.djangoproject.com/en/dev/ref/models/constraints/), then you should aim for that.
Less code to write, less to code to maintain, the database will take care of the data even if it's being inserted from a different place.
Lets look at an example!
```python
class Course(BaseModel):
name = models.CharField(unique=True, max_length=255)
start_date = models.DateField()
end_date = models.DateField()
class Meta:
constraints = [
models.CheckConstraint(
name="start_date_before_end_date",
check=Q(start_date__lt=F("end_date"))
)
]
```
Now, if we try to create new object via `course.save()` or via `Course.objects.create(...)`, we are going to get an `IntegrityError`, rather than a `ValidationError`.
This can actually be a downside to the approach, because now, we have to deal with the `IntegrityError`, which does not always have the best error message.
The Django's documentation on constraints is quite lean, so you can check the following articles by Adam Johnson, for examples of how to use them:
1. [Using Django Check Constraints to Ensure Only One Field Is Set](https://adamj.eu/tech/2020/03/25/django-check-constraints-one-field-set/)
1. [Django’s Field Choices Don’t Constrain Your Data](https://adamj.eu/tech/2020/01/22/djangos-field-choices-dont-constrain-your-data/)
1. [Using Django Check Constraints to Prevent Self-Following](https://adamj.eu/tech/2021/02/26/django-check-constraints-prevent-self-following/)
### Properties
Model properties are great way to quickly access a derived value from a model's instance.
For example, lets look at the `has_started` and `has_finished` properties of our `Course` model:
```python
from django.utils import timezone
from django.core.exceptions import ValidationError
class Course(BaseModel):
name = models.CharField(unique=True, max_length=255)
start_date = models.DateField()
end_date = models.DateField()
def clean(self):
if self.start_date > self.end_date:
raise ValidationError("End date cannot be before start date!")
def save(self, *args, **kwargs):
self.full_clean()
return super().save(*args, **kwargs)
if self.start_date >= self.end_date:
raise ValidationError("End date cannot be before start date")
@property
def visible_teachers(self):
return self.teachers.filter(course_assignments__hidden=False).select_related('profile')
@property
def duration_in_weeks(self):
weeks = rrule.rrule(
rrule.WEEKLY,
dtstart=self.start_date,
until=self.end_date
)
return weeks.count()
@property
def has_started(self):
now = get_now()
def has_started(self) -> bool:
now = timezone.now()
return self.start_date <= now.date()
@property
def has_finished(self):
now = get_now()
def has_finished(self) -> bool:
now = timezone.now()
return self.end_date <= now.date()
@property
def can_generate_certificates(self):
now = get_now()
return now.date() <= self.end_date + self.generate_certificates_delta
def __str__(self) -> str:
return self.name
```
Few things to spot here.
Those properties are handy, because we can now refer to them in serializers or use them in templates.
**Custom validation:**
**We have few general rules of thumb, for when to add properties to the model:**
* There's a custom model validation, defined in `clean()`. This validation uses only model fields and spans no relations.
* This requires someone to call `full_clean()` on the model instance. The best place to do that is in the `save()` method of the model. Otherwise, people can forget to call `full_clean()` in the respective service.
1. If we need a simple derived value, based on **non-relational model fields**, add a `@property` for that.
1. If the calculation of the derived value is simple enough.
**Properties:**
**Properties should be something else (service, selector, utility) in the following cases:**
* All properties, except `visible_teachers`, work directly on model fields.
* `visible_teachers` is a great candidate for a **selector**.
We have few general rules for custom validations & model properties / methods:
### Custom validation
* If the custom validation depends only on the **non-relational model fields**, define it in `clean` and call `full_clean` in `save`.
* If the custom validation is more complex & **spans relationships**, do it in the service that creates the model.
* It's OK to combine both `clean` and additional validation in the `service`.
* As proposed in [this issue](https://github.com/HackSoftware/Django-Styleguide/issues/22), if you can do validation using [Django's constraints](https://docs.djangoproject.com/en/2.2/ref/models/constraints/), then you should aim for that. Less code to write.
### Properties
* If your model properties use only **non-relational model fields**, they are OK to stay as properties.
* If a property, such as `visible_teachers` starts **spanning relationships**, it's better to define a selector for that.
1. If we need to span multiple relations or fetch additional data.
1. If the calculation is more complex.
Keep in mind that those rules are vague, because context is quite often important. Use your best judgement!
### Methods
* If you need a method that updates several fields at once (for example - `created_at` and `created_by` when something happens), you can create a model method that does the job.
* Every model method should be wrapped in a service. There should be no model method calling outside a service.
Model methods are also very powerful tool, that can build on top of properties.
Lets see an example with the `is_within(self, x)` method:
```python
from django.core.exceptions import ValidationError
from django.utils import timezone
class Course(BaseModel):
name = models.CharField(unique=True, max_length=255)
start_date = models.DateField()
end_date = models.DateField()
def clean(self):
if self.start_date >= self.end_date:
raise ValidationError("End date cannot be before start date")
@property
def has_started(self) -> bool:
now = timezone.now()
return self.start_date <= now.date()
@property
def has_finished(self) -> bool:
now = timezone.now()
return self.end_date <= now.date()
def is_within(self, x: date) -> bool:
return self.start_date <= x <= self.end_date
```
`is_within` cannot be a property, because it requires an argument. So it's a method instead.
Another great way for using methods in models is using them for **attribute setting**, when setting one attribute must always be followed by setting another attribute with a derived value.
An example:
```python
from django.utils.crypto import get_random_string
from django.conf import settings
from django.utils import timezone
class Token(BaseModel):
secret = models.CharField(max_length=255, unique=True)
expiry = models.DateTimeField(blank=True, null=True)
def set_new_secret(self):
now = timezone.now()
self.secret = get_random_string(255)
self.expiry = now + settings.TOKEN_EXPIRY_TIMEDELTA
return self
```
Now, we can safely call `set_new_secret`, that'll produce correct values for both `secret` and `expiry`.
**We have few general rules of thumb, for when to add methods to the model:**
1. If we need a simple derived value, that requires arguments, based on **non-relational model fields**, add a method for that.
1. If the calculation of the derived value is simple enough.
1. If setting one attribute always requires setting values to other attributes, use a method for that.
**Models should be something else (service, selector, utility) in the following cases:**
1. If we need to span multiple relations or fetch additional data.
1. If the calculation is more complex.
Keep in mind that those rules are vague, because context is quite often important. Use your best judgement!
### Testing
Models need to be tested only if there's something additional to them - like custom validation or properties.
Models need to be tested only if there's something additional to them - like validation, properties or methods.
If we are strict & don't do custom validation / properties, then we can test the models without actually writing anything to the database => we are going to get quicker tests.
For example, if we want to test the custom validation, here's how a test could look like:
Here's an example:
```python
from datetime import timedelta
from django.test import TestCase
from django.core.exceptions import ValidationError
from django.utils import timezone
from project.common.utils import get_now
from project.education.factories import CourseFactory
from project.education.models import Course
from project.some_app.models import Course
class CourseTests(TestCase):
def test_course_end_date_cannot_be_before_start_date(self):
start_date = get_now()
end_date = get_now() - timedelta(days=1)
start_date = timezone.now()
end_date = timezone.now() - timedelta(days=1)
course_data = CourseFactory.build()
course_data['start_date'] = start_date
course_data['end_date'] = end_date
course = Course(**course_data)
course = Course(start_date=start_date, end_date=end_date)
with self.assertRaises(ValidationError):
course.full_clean()
```
There's a lot going on in this test:
A few things to note here:
* `get_now()` returns a timezone aware datetime.
* `CourseFactory.build()` will return a dictionary with all required fields for a course to exist.
* We replace the values for `start_date` and `end_date`.
* We assert that a validation error is going to be raised if we call `full_clean`.
* We are not hitting the database at all, since there's no need for that.
Here's how `CourseFactory` looks like:
```python
class CourseFactory(factory.DjangoModelFactory):
name = factory.Sequence(lambda n: f'{n}{faker.word()}')
start_date = factory.LazyAttribute(
lambda _: get_now()
)
end_date = factory.LazyAttribute(
lambda _: get_now() + timedelta(days=30)
)
slug_url = factory.Sequence(lambda n: f'{n}{faker.slug()}')
repository = factory.LazyAttribute(lambda _: faker.url())
video_channel = factory.LazyAttribute(lambda _: faker.url())
facebook_group = factory.LazyAttribute(lambda _: faker.url())
class Meta:
model = Course
@classmethod
def _build(cls, model_class, *args, **kwargs):
return kwargs
@classmethod
def _create(cls, model_class, *args, **kwargs):
return create_course(**kwargs)
```
1. We assert that a validation error is going to be raised if we call `full_clean`.
1. **We are not hitting the database at all**, since there's no need for that. This can speed up certain tests.
## Services