Fixed #19425 - Added InlineModelAdmin.get_extra hook.

Thanks dave@ for the suggestion and Rohan Jain for the patch.

django/contrib/admin/options.py

@@ -1512,6 +1512,10 @@ class InlineModelAdmin(BaseModelAdmin):
             js.extend(['SelectBox.js', 'SelectFilter2.js'])
         return forms.Media(js=[static('admin/js/%s' % url) for url in js])
 
+    def get_extra(self, request, obj=None, **kwargs):
+        """Hook for customizing the number of extra inline forms."""
+        return self.extra
+
     def get_formset(self, request, obj=None, **kwargs):
         """Returns a BaseInlineFormSet class for use in admin add/change views."""
         if self.declared_fieldsets:
@@ -1538,7 +1542,7 @@ class InlineModelAdmin(BaseModelAdmin):
             "fields": fields,
             "exclude": exclude,
             "formfield_callback": partial(self.formfield_for_dbfield, request=request),
-            "extra": self.extra,
+            "extra": self.get_extra(request, obj, **kwargs),
             "max_num": self.max_num,
             "can_delete": can_delete,
         }

docs/ref/contrib/admin/index.txt

@@ -1715,6 +1715,11 @@ The ``InlineModelAdmin`` class adds:
     The dynamic link will not appear if the number of currently displayed forms
     exceeds ``max_num``, or if the user does not have JavaScript enabled.
 
+    .. versionadded:: 1.6
+
+    :meth:`InlineModelAdmin.get_extra` also allows you to customize the number
+    of extra forms.
+
     .. _ref-contrib-admin-inline-max-num:
 
 .. attribute:: InlineModelAdmin.max_num
@@ -1762,6 +1767,26 @@ The ``InlineModelAdmin`` class adds:
     Returns a ``BaseInlineFormSet`` class for use in admin add/change views.
     See the example for :class:`ModelAdmin.get_formsets`.
 
+.. method:: InlineModelAdmin.get_extra(self, request, obj=None, **kwargs)
+
+    .. versionadded:: 1.6
+
+    Returns the number of extra inline forms to use. By default, returns the
+    :attr:`InlineModelAdmin.extra` attribute.
+
+    Override this method to programmatically determine the number of extra
+    inline forms. For example, this may be based on the model instance
+    (passed as the keyword argument ``obj``)::
+
+        class BinaryTreeAdmin(admin.TabularInline):
+            model = BinaryTree
+
+            def get_extra(self, request, obj=None, **kwargs):
+                extra = 2
+                if obj:
+                    return extra - obj.binarytree_set.count()
+                return extra
+
 Working with a model with two or more foreign keys to the same parent model
 ---------------------------------------------------------------------------
 

docs/releases/1.6.txt

@@ -292,6 +292,10 @@ Minor features
   :meth:`~django.db.models.query.QuerySet.select_related` can be cleared using
   ``select_related(None)``.
 
+* The :meth:`~django.contrib.admin.InlineModelAdmin.get_extra` method on
+  :class:`~django.contrib.admin.InlineModelAdmin` may be overridden to
+  customize the number of extra inline forms.
+
 Backwards incompatible changes in 1.6
 =====================================
 

tests/admin_inlines/admin.py

@@ -129,6 +129,17 @@ class ChildModel1Inline(admin.TabularInline):
 class ChildModel2Inline(admin.StackedInline):
     model = ChildModel2
 
+# admin for #19425
+class BinaryTreeAdmin(admin.TabularInline):
+    model = BinaryTree
+
+    def get_extra(self, request, obj=None, **kwargs):
+        extra = 2
+        if obj:
+            return extra - obj.binarytree_set.count()
+
+        return extra
+
 # admin for #19524
 class SightingInline(admin.TabularInline):
     model = Sighting
@@ -150,4 +161,5 @@ site.register(Author, AuthorAdmin)
 site.register(CapoFamiglia, inlines=[ConsigliereInline, SottoCapoInline, ReadOnlyInlineInline])
 site.register(ProfileCollection, inlines=[ProfileInline])
 site.register(ParentModelWithCustomPk, inlines=[ChildModel1Inline, ChildModel2Inline])
+site.register(BinaryTree, inlines=[BinaryTreeAdmin])
 site.register(ExtraTerrestrial, inlines=[SightingInline])

tests/admin_inlines/models.py

@@ -183,6 +183,12 @@ class ChildModel2(models.Model):
     def get_absolute_url(self):
         return '/child_model2/'
 
+
+# Models for #19425
+class BinaryTree(models.Model):
+    name = models.CharField(max_length=100)
+    parent = models.ForeignKey('self', null=True, blank=True)
+
 # Models for #19524
 
 class LifeForm(models.Model):

tests/admin_inlines/tests.py

@@ -12,7 +12,7 @@ from .admin import InnerInline, TitleInline, site
 from .models import (Holder, Inner, Holder2, Inner2, Holder3, Inner3, Person,
     OutfitItem, Fashionista, Teacher, Parent, Child, Author, Book, Profile,
     ProfileCollection, ParentModelWithCustomPk, ChildModel1, ChildModel2,
-    Sighting, Title, Novel, Chapter, FootNote)
+    Sighting, Title, Novel, Chapter, FootNote, BinaryTree)
 
 
 @override_settings(PASSWORD_HASHERS=('django.contrib.auth.hashers.SHA1PasswordHasher',))
@@ -193,6 +193,18 @@ class TestInline(TestCase):
         self.assertEqual(response.status_code, 302)
         self.assertEqual(Sighting.objects.filter(et__name='Martian').count(), 1)
 
+    def test_custom_get_extra_form(self):
+        bt_head = BinaryTree.objects.create(name="Tree Head")
+        bt_child = BinaryTree.objects.create(name="First Child", parent=bt_head)
+
+        # The total number of forms will remain the same in either case
+        total_forms_hidden = '<input id="id_binarytree_set-TOTAL_FORMS" name="binarytree_set-TOTAL_FORMS" type="hidden" value="2" />'
+        response = self.client.get('/admin/admin_inlines/binarytree/add/')
+        self.assertContains(response, total_forms_hidden)
+
+        response = self.client.get("/admin/admin_inlines/binarytree/%d/" % bt_head.id)
+        self.assertContains(response, total_forms_hidden)
+
 
 @override_settings(PASSWORD_HASHERS=('django.contrib.auth.hashers.SHA1PasswordHasher',))
 class TestInlineMedia(TestCase):