wagtail/docs/reference/pages/theory.md

(pages_theory)=

# Theory

## Introduction to Trees

If you're unfamiliar with trees as an abstract data type, you might want to [review the concepts involved](<https://en.wikipedia.org/wiki/Tree_(data_structure)>).

As a web developer, though, you probably already have a good understanding of trees as filesystem directories or paths. Wagtail pages can create the same structure, as each page in the tree has its own URL path, like so:

```
/
    people/
        nien-nunb/
        laura-roslin/
    events/
        captain-picard-day/
        winter-wrap-up/
```

The Wagtail admin interface uses the tree to organise content for editing, letting you navigate up and down levels in the tree through its Explorer menu. This method of organisation is a good place to start in thinking about your own Wagtail models.

### Nodes and Leaves

It might be handy to think of the `Page`-derived models you want to create as being one of two node types: parents and leaves. Wagtail isn't prescriptive in this approach, but it's a good place to start if you're not experienced in structuring your own content types.

#### Nodes

Parent nodes on the Wagtail tree probably want to organise and display a browse-able index of their descendants. A blog, for instance, needs a way to show a list of individual posts.

A Parent node could provide its own function returning its descendant objects.

```python
class EventPageIndex(Page):
    # ...
    def events(self):
        # Get list of live event pages that are descendants of this page
        events = EventPage.objects.live().descendant_of(self)

        # Filter events list to get ones that are either
        # running now or start in the future
        events = events.filter(date_from__gte=date.today())

        # Order by date
        events = events.order_by('date_from')

        return events
```

This example makes sure to limit the returned objects to pieces of content which make sense, specifically ones which have been published through Wagtail's admin interface (`live()`) and are children of this node (`descendant_of(self)`). By setting a `subpage_types` class property in your model, you can specify which models are allowed to be set as children, and by setting a `parent_page_types` class property, you can specify which models are allowed to be parents of this page model. Wagtail will allow any `Page`-derived model by default. Regardless, it's smart for a parent model to provide an index filtered to make sense.

#### Leaves

Leaves are the pieces of content itself, a page which is consumable, and might just consist of a bunch of properties. A blog page leaf might have some body text and an image. A person page leaf might have a photo, a name, and an address.

It might be helpful for a leaf to provide a way to back up along the tree to a parent, such as in the case of breadcrumbs navigation. The tree might also be deep enough that a leaf's parent won't be included in general site navigation.

The model for the leaf could provide a function that traverses the tree in the opposite direction and returns an appropriate ancestor:

```python
class EventPage(Page):
    # ...
    def event_index(self):
        # Find closest ancestor which is an event index
        return self.get_ancestors().type(EventIndexPage).last()
```

If defined, `subpage_types` and `parent_page_types` will also limit the parent models allowed to contain a leaf. If not, Wagtail will allow any combination of parents and leafs to be associated in the Wagtail tree. Like with index pages, it's a good idea to make sure that the index is actually of the expected model to contain the leaf.

#### Other Relationships

Your `Page`-derived models might have other interrelationships which extend the basic Wagtail tree or depart from it entirely. You could provide functions to navigate between siblings, such as a "Next Post" link on a blog page (`post->post->post`). It might make sense for subtrees to interrelate, such as in a discussion forum (`forum->post->replies`) Skipping across the hierarchy might make sense, too, as all objects of a certain model class might interrelate regardless of their ancestors (`events = EventPage.objects.all`). It's largely up to the models to define their interrelations, the possibilities are really endless.

(anatomy_of_a_wagtail_request)=

## Anatomy of a Wagtail Request

For going beyond the basics of model definition and interrelation, it might help to know how Wagtail handles requests and constructs responses. In short, it goes something like:

1.  Django gets a request and routes through Wagtail's URL dispatcher definitions
2.  Wagtail checks the hostname of the request to determine which `Site` record will handle this request.
3.  Starting from the root page of that site, Wagtail traverses the page tree, calling the `route()` method and letting each page model decide whether it will handle the request itself or pass it on to a child page.
4.  The page responsible for handling the request returns a `RouteResult` object from `route()`, which identifies the page along with any additional `args`/`kwargs` to be passed to `serve()`.
5.  Wagtail calls `serve()`, which constructs a context using `get_context()`
6.  `serve()` finds a template to pass it to using `get_template()`
7.  A response object is returned by `serve()` and Django responds to the requester.

You can apply custom behaviour to this process by overriding `Page` class methods such as `route()` and `serve()` in your own models. For examples, see [Recipes](page_model_recipes).

(scheduled_publishing)=

## Scheduled Publishing

Page publishing can be scheduled through the _Go live date/time_ feature in the _Settings_ tab of the _Edit_ page. This allows you to set set up initial page publishing or a page update in advance.
In order for pages to be published at the scheduled time you should set up the [publish_scheduled_pages](publish_scheduled_pages) management command.

The basic workflow is as follows:

-   Scheduling a revision for a page that is not currently live means that page will go live when the scheduled time comes.
-   Scheduling a revision for a page that is already live means that revision will be published when the time comes.
-   If page has a scheduled revision and you set another revision to publish immediately, the scheduled revision will be unscheduled.

The _Revisions_ view for a given page will show which revision is scheduled and when it is scheduled for. A scheduled revision in the list will also provide an _Unschedule_ button to cancel it.