Fixed #18388 - Added InlineModelAdmin.get_max_num hook.

Thanks d.willy.c.c@ for the suggestion and Melevir and Areski Belaid for work
on the patch.

django/contrib/admin/options.py

@@ -1516,6 +1516,10 @@ class InlineModelAdmin(BaseModelAdmin):
         """Hook for customizing the number of extra inline forms."""
         return self.extra
 
+    def get_max_num(self, request, obj=None, **kwargs):
+        """Hook for customizing the max number of extra inline forms."""
+        return self.max_num
+
     def get_formset(self, request, obj=None, **kwargs):
         """Returns a BaseInlineFormSet class for use in admin add/change views."""
         if 'fields' in kwargs:
@@ -1543,7 +1547,7 @@ class InlineModelAdmin(BaseModelAdmin):
             "exclude": exclude,
             "formfield_callback": partial(self.formfield_for_dbfield, request=request),
             "extra": self.get_extra(request, obj, **kwargs),
-            "max_num": self.max_num,
+            "max_num": self.get_max_num(request, obj, **kwargs),
             "can_delete": can_delete,
         }
 

docs/ref/contrib/admin/index.txt

@@ -1728,6 +1728,11 @@ The ``InlineModelAdmin`` class adds:
     doesn't directly correlate to the number of objects, but can if the value
     is small enough. See :ref:`model-formsets-max-num` for more information.
 
+    .. versionadded:: 1.6
+
+    :meth:`InlineModelAdmin.get_max_num` also allows you to customize the
+    maximum number of extra forms.
+
 .. attribute:: InlineModelAdmin.raw_id_fields
 
     By default, Django's admin uses a select-box interface (<select>) for
@@ -1787,6 +1792,27 @@ The ``InlineModelAdmin`` class adds:
                     return extra - obj.binarytree_set.count()
                 return extra
 
+.. method:: InlineModelAdmin.get_max_num(self, request, obj=None, **kwargs)
+
+    .. versionadded:: 1.6
+
+    Returns the maximum number of extra inline forms to use. By default,
+    returns the :attr:`InlineModelAdmin.max_num` attribute.
+
+    Override this method to programmatically determine the maximum number of
+    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_max_num(self, request, obj=None, **kwargs):
+                max_num = 10
+                if obj.parent:
+                    return max_num - 5
+                return max_num
+
+
 Working with a model with two or more foreign keys to the same parent model
 ---------------------------------------------------------------------------
 

docs/releases/1.6.txt

@@ -292,9 +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
+* The :meth:`~django.contrib.admin.InlineModelAdmin.get_extra` and
+  :meth:`~django.contrib.admin.InlineModelAdmin.get_max_num` methods on
   :class:`~django.contrib.admin.InlineModelAdmin` may be overridden to
-  customize the number of extra inline forms.
+  customize the extra and maximum number of inline forms.
 
 Backwards incompatible changes in 1.6
 =====================================

tests/admin_inlines/admin.py

@@ -129,7 +129,7 @@ class ChildModel1Inline(admin.TabularInline):
 class ChildModel2Inline(admin.StackedInline):
     model = ChildModel2
 
-# admin for #19425
+# admin for #19425 and #18388
 class BinaryTreeAdmin(admin.TabularInline):
     model = BinaryTree
 
@@ -137,9 +137,14 @@ class BinaryTreeAdmin(admin.TabularInline):
         extra = 2
         if obj:
             return extra - obj.binarytree_set.count()
-
         return extra
 
+    def get_max_num(self, request, obj=None, **kwargs):
+        max_num = 3
+        if obj:
+            return max_num - obj.binarytree_set.count()
+        return max_num
+
 # admin for #19524
 class SightingInline(admin.TabularInline):
     model = Sighting

tests/admin_inlines/tests.py

@@ -197,12 +197,18 @@ class TestInline(TestCase):
         bt_head = BinaryTree.objects.create(name="Tree Head")
         bt_child = BinaryTree.objects.create(name="First Child", parent=bt_head)
 
+        # The maximum number of forms should respect 'get_max_num' on the
+        # ModelAdmin
+        max_forms_input = '<input id="id_binarytree_set-MAX_NUM_FORMS" name="binarytree_set-MAX_NUM_FORMS" type="hidden" value="%d" />'
         # 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, max_forms_input % 3)
         self.assertContains(response, total_forms_hidden)
 
         response = self.client.get("/admin/admin_inlines/binarytree/%d/" % bt_head.id)
+        self.assertContains(response, max_forms_input % 2)
         self.assertContains(response, total_forms_hidden)