
.. _model_recipes:

Recipes
=======

Overriding the :meth:`~wagtail.core.models.Page.serve` Method
--------------------------------------------------------------------

Wagtail defaults to serving :class:`~wagtail.core.models.Page`-derived models by passing a reference to the page object to a Django HTML template matching the model's name, but suppose you wanted to serve something other than HTML? You can override the :meth:`~wagtail.core.models.Page.serve` method provided by the :class:`~wagtail.core.models.Page` class and handle the Django request and response more directly.

Consider this example from the Wagtail demo site's ``models.py``, which serves an ``EventPage`` object as an iCal file if the ``format`` variable is set in the request:

.. code-block:: python

    class EventPage(Page):
        ...

        def serve(self, request):
            if "format" in request.GET:
                if request.GET['format'] == 'ical':
                    # Export to ical format
                    response = HttpResponse(
                        export_event(self, 'ical'),
                        content_type='text/calendar',
                    )
                    response['Content-Disposition'] = 'attachment; filename=' + self.slug + '.ics'
                    return response
                else:
                    # Unrecognised format error
                    message = 'Could not export event\n\nUnrecognised format: ' + request.GET['format']
                    return HttpResponse(message, content_type='text/plain')
            else:
                # Display event page as usual
                return super().serve(request)

:meth:`~wagtail.core.models.Page.serve` takes a Django request object and returns a Django response object. Wagtail returns a ``TemplateResponse`` object with the template and context which it generates, which allows middleware to function as intended, so keep in mind that a simpler response object like a ``HttpResponse`` will not receive these benefits.

With this strategy, you could use Django or Python utilities to render your model in JSON or XML or any other format you'd like.


.. _overriding_route_method:

Adding Endpoints with Custom :meth:`~wagtail.core.models.Page.route` Methods
-----------------------------------------------------------------------------------

.. note::

    A much simpler way of adding more endpoints to pages is provided by the :mod:`~wagtail.contrib.routable_page` module.

Wagtail routes requests by iterating over the path components (separated with a forward slash ``/``), finding matching objects based on their slug, and delegating further routing to that object's model class. The Wagtail source is very instructive in figuring out what's happening. This is the default ``route()`` method of the ``Page`` class:

.. code-block:: python

    class Page(...):
        ...

        def route(self, request, path_components):
            if path_components:
                # request is for a child of this page
                child_slug = path_components[0]
                remaining_components = path_components[1:]

                # find a matching child or 404
                try:
                    subpage = self.get_children().get(slug=child_slug)
                except Page.DoesNotExist:
                    raise Http404

                # delegate further routing
                return subpage.specific.route(request, remaining_components)

            else:
                # request is for this very page
                if self.live:
                    # Return a RouteResult that will tell Wagtail to call
                    # this page's serve() method
                    return RouteResult(self)
                else:
                    # the page matches the request, but isn't published, so 404
                    raise Http404

:meth:`~wagtail.core.models.Page.route` takes the current object (``self``), the ``request`` object, and a list of the remaining ``path_components`` from the request URL. It either continues delegating routing by calling :meth:`~wagtail.core.models.Page.route` again on one of its children in the Wagtail tree, or ends the routing process by returning a ``RouteResult`` object or raising a 404 error.

The ``RouteResult`` object (defined in wagtail.core.url_routing) encapsulates all the information Wagtail needs to call a page's :meth:`~wagtail.core.models.Page.serve` method and return a final response: this information consists of the page object, and any additional ``args``/``kwargs`` to be passed to :meth:`~wagtail.core.models.Page.serve`.

By overriding the :meth:`~wagtail.core.models.Page.route` method, we could create custom endpoints for each object in the Wagtail tree. One use case might be using an alternate template when encountering the ``print/`` endpoint in the path. Another might be a REST API which interacts with the current object. Just to see what's involved, lets make a simple model which prints out all of its child path components.

First, ``models.py``:

.. code-block:: python

    from django.shortcuts import render
    from wagtail.core.url_routing import RouteResult
    from django.http.response import Http404
    from wagtail.core.models import Page

    ...

    class Echoer(Page):

        def route(self, request, path_components):
            if path_components:
                # tell Wagtail to call self.serve() with an additional 'path_components' kwarg
                return RouteResult(self, kwargs={'path_components': path_components})
            else:
                if self.live:
                    # tell Wagtail to call self.serve() with no further args
                    return RouteResult(self)
                else:
                    raise Http404

        def serve(self, path_components=[]):
            return render(request, self.template, {
                'page': self,
                'echo': ' '.join(path_components),
            })


This model, ``Echoer``, doesn't define any properties, but does subclass ``Page`` so objects will be able to have a custom title and slug. The template just has to display our ``{{ echo }}`` property.

Now, once creating a new ``Echoer`` page in the Wagtail admin titled "Echo Base," requests such as::

    http://127.0.0.1:8000/echo-base/tauntaun/kennel/bed/and/breakfast/

Will return::

    tauntaun kennel bed and breakfast

Be careful if you're introducing new required arguments to the ``serve()`` method - Wagtail still needs to be able to display a default view of the page for previewing and moderation, and by default will attempt to do this by calling ``serve()`` with a request object and no further arguments. If your ``serve()`` method does not accept that as a method signature, you will need to override the page's ``serve_preview()`` method to call ``serve()`` with suitable arguments:

.. code-block:: python

    def serve_preview(self, request, mode_name):
        return self.serve(request, color='purple')

.. _tagging:

Tagging
-------

Wagtail provides tagging capability through the combination of two django modules, ``taggit`` and ``modelcluster``. ``taggit`` provides a model for tags which is extended by ``modelcluster``, which in turn provides some magical database abstraction which makes drafts and revisions possible in Wagtail. It's a tricky recipe, but the net effect is a many-to-many relationship between your model and a tag class reserved for your model.

Using an example from the Wagtail demo site, here's what the tag model and the relationship field looks like in ``models.py``:

.. code-block:: python

    from modelcluster.fields import ParentalKey
    from modelcluster.contrib.taggit import ClusterTaggableManager
    from taggit.models import TaggedItemBase

    class BlogPageTag(TaggedItemBase):
        content_object = ParentalKey('demo.BlogPage', on_delete=models.CASCADE, related_name='tagged_items')

    class BlogPage(Page):
        ...
        tags = ClusterTaggableManager(through=BlogPageTag, blank=True)

        promote_panels = Page.promote_panels + [
            ...
            FieldPanel('tags'),
        ]

Wagtail's admin provides a nice interface for inputting tags into your content, with typeahead tag completion and friendly tag icons.

Now that we have the many-to-many tag relationship in place, we can fit in a way to render both sides of the relation. Here's more of the Wagtail demo site ``models.py``, where the index model for ``BlogPage`` is extended with logic for filtering the index by tag:

.. code-block:: python

    from django.shortcuts import render

    class BlogIndexPage(Page):
        ...
        def serve(self, request):
            # Get blogs
            blogs = BlogPage.objects.child_of(self).live()

            # Filter by tag
            tag = request.GET.get('tag')
            if tag:
                blogs = blogs.filter(tags__name=tag)

            return render(request, self.template, {
                'page': self,
                'blogs': blogs,
            })

Here, ``blogs.filter(tags__name=tag)`` invokes a reverse Django QuerySet filter on the ``BlogPageTag`` model to optionally limit the ``BlogPage`` objects sent to the template for rendering. Now, lets render both sides of the relation by showing the tags associated with an object and a way of showing all of the objects associated with each tag. This could be added to the ``blog_page.html`` template:

.. code-block:: html+django

    {% for tag in page.tags.all %}
        <a href="{% pageurl page.blog_index %}?tag={{ tag }}">{{ tag }}</a>
    {% endfor %}

Iterating through ``page.tags.all`` will display each tag associated with ``page``, while the link(s) back to the index make use of the filter option added to the ``BlogIndexPage`` model. A Django query could also use the ``tagged_items`` related name field to get ``BlogPage`` objects associated with a tag.

This is just one possible way of creating a taxonomy for Wagtail objects. With all of the components for a taxonomy available through Wagtail, you should be able to fulfil even the most exotic taxonomic schemes.
