Add Using Inline Formsets

django/using_inline_formsets.md

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+# Using inline formsets with `inlineformset_factory`
+
+## Links 
+
+- [Creating forms from models | Django documentation | Django 5.0](https://docs.djangoproject.com/en/5.0/topics/forms/modelforms/#inline-formsets)
+
+I'm going to let the Django docs do a lot of the heavy lifting in this TIL and quote them liberally: 
+
+## Simplest example 
+
+Assume these models: 
+
+```python
+from django.db import models
+
+
+class Author(models.Model):
+    name = models.CharField(max_length=100)
+
+
+class Book(models.Model):
+    author = models.ForeignKey(Author, on_delete=models.CASCADE)
+    title = models.CharField(max_length=100)
+```
+
+> If you want to create a formset that allows you to edit books belonging to a particular author, you could do this:
+
+
+```bash
+>>> from django.forms import inlineformset_factory
+>>> BookFormSet = inlineformset_factory(Author, Book, fields=["title"])
+>>> author = Author.objects.get(name="Mike Royko")
+>>> formset = BookFormSet(instance=author)
+```
+
+
+## Scenario: I need to change the behavior of the `InlineFormSet` 
+
+Subclass `BaseInlineFormSet` and override the method: 
+
+```python
+from django.forms import BaseInlineFormSet
+
+
+class CustomInlineFormSet(BaseInlineFormSet):
+    def clean(self):
+        super().clean()
+        # example custom validation across forms in the formset
+        for form in self.forms:
+            # your custom formset validation
+            ...
+```
+
+Then, when creating the formset, pass your custom formset as an argument to `inlineformset_factory`: 
+
+```bash
+>>> from django.forms import inlineformset_factory
+>>> BookFormSet = inlineformset_factory(
+...     Author, Book, fields=["title"], formset=CustomInlineFormSet
+... )
+>>> author = Author.objects.get(name="Mike Royko")
+>>> formset = BookFormSet(instance=author)
+```
+
+## Scenario: Your model contains for than one foreign key field to the same model 
+
+Example: a `Friendship` model: 
+
+```python
+class Friendship(models.Model):
+    from_friend = models.ForeignKey(
+        Friend,
+        on_delete=models.CASCADE,
+        related_name="from_friends",
+    )
+    to_friend = models.ForeignKey(
+        Friend,
+        on_delete=models.CASCADE,
+        related_name="friends",
+    )
+    length_in_months = models.IntegerField()
+```
+
+To solve, pass the `fk_name` argument to `inlineformset_factory`: 
+
+```bash
+>>> FriendshipFormSet = inlineformset_factory(
+...     Friend, Friendship, fk_name="from_friend", fields=["to_friend", "length_in_months"]
+... )
+```
+
+## Scenario: Use the inline formset in a view 
+
+
+```python
+def manage_books(request, author_id):
+    author = Author.objects.get(pk=author_id)
+    BookInlineFormSet = inlineformset_factory(Author, Book, fields=["title"])
+    if request.method == "POST":
+        formset = BookInlineFormSet(request.POST, request.FILES, instance=author)
+        if formset.is_valid():
+            formset.save()
+            # Do something. Should generally end with a redirect. For example:
+            return HttpResponseRedirect(author.get_absolute_url())
+    else:
+        formset = BookInlineFormSet(instance=author)
+    return render(request, "manage_books.html", {"formset": formset})
+```