Try writing an admin action to clone a brew...

reinheit/apps/brew/admin.py

@@ -44,3 +44,31 @@ class BrewAdmin(admin.ModelAdmin):
 
     list_display = ["name", "style", "pitch_date"]
     date_hierarchy = "pitch_date"
+
+    actions = ["clone"]
+
+    @admin.action(description="I brewed another one of these")
+    def clone(self, request, queryset):
+        for brew in queryset:
+            malt_additions = brew.maltaddition_set.all()
+            fermentable_additions = brew.fermentableaddition_set.all()
+            hop_additions = brew.hopaddition_set.all()
+            chemical_additions = brew.chemicaladdition_set.all()
+            yeast_additions = brew.yeastaddition_set.all()
+
+            brew.pk = None
+            brew.save()
+
+            for add_type in (
+                malt_additions,
+                fermentable_additions,
+                hop_additions,
+                chemical_additions,
+                yeast_additions
+            ):
+                for addition in add_type:
+                    addition.pk = None
+                    addition.brew_id = brew.pk
+                    addition.save()
+
+