Go – Filtering models with inheritance in Django

djangodjango-models

I have two Django model classes that are structured similar to the following:

class Build(models.Model):
    project = models.CharField(max_length=100)
    ...

class CustomBuild(Build):
    custom_type = ...
    ...

I want to select all Builds and CustomBuilds (each CustomBuild has a one-to-one relationship with a Build) from the database with a specific project attribute.

I believe Build.objects.filter(project="myproject") will select the correct objects but many of them will be missing the additional data (such as custom_type) that would be provided by a CustomBuild object. On the other hand, filtering CustomBuild.objects will exclude those objects that are not CustomBuilds.

How can I accomplish this? Thanks.

Best Answer

You can fetch Build objects with Build.objects.filter() and access the subclass when you need to:

qs = Build.objects.all()
build_obj = qs[4]
custom_build_obj = build_obj.custom_build
custom_build_obj.custom_type = ...

Related Solutions

Python – a “slug” in Django

A "slug" is a way of generating a valid URL, generally using data already obtained. For instance, a slug uses the title of an article to generate a URL. I advise to generate the slug by means of a function, given the title (or another piece of data), rather than setting it manually.

An example:

<title> The 46 Year Old Virgin </title>
<content> A silly comedy movie </content>
<slug> the-46-year-old-virgin </slug>

Now let's pretend that we have a Django model such as:

class Article(models.Model):
    title = models.CharField(max_length=100)
    content = models.TextField(max_length=1000)
    slug = models.SlugField(max_length=40)

How would you reference this object with a URL and with a meaningful name? You could for instance use Article.id so the URL would look like this:

www.example.com/article/23

Or, you might want to reference the title like this:

www.example.com/article/The 46 Year Old Virgin

Since spaces aren't valid in URLs, they must be replaced by %20, which results in:

www.example.com/article/The%2046%20Year%20Old%20Virgin

Both attempts are not resulting in very meaningful, easy-to-read URL. This is better:

www.example.com/article/the-46-year-old-virgin

In this example, the-46-year-old-virgin is a slug: it is created from the title by down-casing all letters, and replacing spaces by hyphens -.

Also see the URL of this very web page for another example.

Python – How to do a not equal in Django queryset filtering

You can use Q objects for this. They can be negated with the ~ operator and combined much like normal Python expressions:

from myapp.models import Entry
from django.db.models import Q

Entry.objects.filter(~Q(id=3))

will return all entries except the one(s) with 3 as their ID:

[<Entry: Entry object>, <Entry: Entry object>, <Entry: Entry object>, ...]

Best Answer

Related Solutions

Python – a “slug” in Django

Python – How to do a not equal in Django queryset filtering

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