[FEAT] Auto select existing user when an account already exists (#4325)

* feat :star: auto select existing user when an account already exists

* lint

* Added test and simplyfied code

---------

Co-authored-by: Lars van de Kerkhof <lars@permanentmarkers.nl>

src/oscar/apps/checkout/forms.py

@@ -84,6 +84,13 @@ class GatewayForm(AuthenticationForm):
                 if User._default_manager.filter(email__iexact=email).exists():
                     msg = _("A user with that email address already exists")
                     self._errors["username"] = self.error_class([msg])
+
+                    # change the value of the 'options' in the submitted data,
+                    # which will be used to render the form
+                    options_field = self.add_prefix("options")
+                    self.data = self.data.copy()
+                    self.data[options_field] = self.EXISTING
+
             return self.cleaned_data
         return super().clean()
 

tests/functional/checkout/test_guest_checkout.py

@@ -107,6 +107,29 @@ class TestIndexView(
         page = self.get(reverse("checkout:index"))
         self.assertEqual(email, page.form["username"].value)
 
+    def test_auto_select_existing_user(self):
+        email = "forgetfulguest@test.com"
+        self.create_user(email, email, self.password)
+
+        self.add_product_to_basket()
+
+        # select guest checkout
+        page = self.get(reverse("checkout:index"))
+        form = page.form
+        form["options"].select(GatewayForm.GUEST)
+        form["username"].value = email
+
+        response = form.submit()
+
+        # since this user allready exists
+        self.assertEqual(email, response.form["username"].value)
+        self.assertEqual(
+            GatewayForm.EXISTING,
+            response.form["options"].value,
+            "Since this user has an account, the GatewayForm should "
+            "be changed to existing, so the user can enter his password",
+        )
+
 
 @override_settings(OSCAR_ALLOW_ANON_CHECKOUT=True)
 class TestShippingAddressView(