Add a dummy_request method to Page so that we can generate page previews without passing in the active request object (which may have undesirable side effects, e.g. may be a POST)

wagtail/wagtailadmin/views/pages.py

@@ -340,12 +340,14 @@ def preview_on_edit(request, page_id):
     if form.is_valid():
         form.save(commit=False)
 
-        # FIXME: passing the original request to page.serve is dodgy (particularly if page.serve has
-        # special treatment of POSTs). Ought to construct one that more or less matches what would be sent
-        # as a front-end GET request
+        # This view will generally be invoked as an AJAX request; as such, in the case of
+        # an error Django will return a plaintext response. This isn't what we want, since
+        # we will be writing the response back to an HTML page regardless of success or
+        # failure - as such, we strip out the X-Requested-With header to get Django to return
+        # an HTML error response
+        request.META.pop('HTTP_X_REQUESTED_WITH', None)
 
-        request.META.pop('HTTP_X_REQUESTED_WITH', None)  # Make this request appear to the page's serve method as a non-ajax one, as they will often implement custom behaviour for XHR
-        response = page.serve(request)
+        response = page.serve(page.dummy_request())
 
         response['X-Wagtail-Preview'] = 'ok'
         return response
@@ -380,10 +382,14 @@ def preview_on_create(request, content_type_app_name, content_type_model_name, p
     if form.is_valid():
         form.save(commit=False)
 
-        # FIXME: passing the original request to page.serve is dodgy (particularly if page.serve has
-        # special treatment of POSTs). Ought to construct one that more or less matches what would be sent
-        # as a front-end GET request
-        response = page.serve(request)
+        # This view will generally be invoked as an AJAX request; as such, in the case of
+        # an error Django will return a plaintext response. This isn't what we want, since
+        # we will be writing the response back to an HTML page regardless of success or
+        # failure - as such, we strip out the X-Requested-With header to get Django to return
+        # an HTML error response
+        request.META.pop('HTTP_X_REQUESTED_WITH', None)
+
+        response = page.serve(page.dummy_request())
 
         response['X-Wagtail-Preview'] = 'ok'
         return response

wagtail/wagtailcore/models.py

@@ -1,5 +1,7 @@
 import sys
 import os
+from StringIO import StringIO
+from urlparse import urlparse
 
 from modelcluster.models import ClusterableModel
 
@@ -7,6 +9,8 @@ from django.db import models, connection, transaction
 from django.db.models import get_model, Q
 from django.http import Http404
 from django.core.cache import cache
+from django.core.handlers.wsgi import WSGIRequest
+from django.core.handlers.base import BaseHandler
 from django.contrib.contenttypes.models import ContentType
 from django.contrib.auth.models import Group
 from django.conf import settings
@@ -501,6 +505,41 @@ class Page(MP_Node, ClusterableModel, Indexed):
         user_perms = UserPagePermissionsProxy(user)
         return user_perms.for_page(self)
 
+    def dummy_request(self):
+        """
+        Construct a HttpRequest object that is, as far as possible, representative of ones that would
+        receive this page as a response. Used for previewing / moderation and any other place where we
+        want to display a view of this page in the admin interface without going through the regular
+        page routing logic.
+        """
+        url = self.full_url
+        if url:
+            url_info = urlparse(url)
+            hostname = url_info.netloc
+            path = url_info.path
+            port = url_info.port or 80
+        else:
+            hostname = 'example.com'
+            path = '/'
+            port = 80
+
+        request = WSGIRequest({
+            'REQUEST_METHOD': 'GET',
+            'PATH_INFO': path,
+            'SERVER_NAME': hostname,
+            'SERVER_PORT': port,
+            'wsgi.input': StringIO(),
+        })
+
+        # Apply middleware to the request - see http://www.mellowmorning.com/2011/04/18/mock-django-request-for-testing/
+        handler = BaseHandler()
+        handler.load_middleware()
+        for middleware_method in handler._request_middleware:
+            if middleware_method(request):
+                raise Exception("Couldn't create request mock object - "
+                                "request middleware returned a response")
+        return request
+
 
 def get_navigation_menu_items():
     # Get all pages that appear in the navigation menu: ones which have children,