Add docs for DRF-based choosers

docs/extending/generic_views.md

@@ -77,3 +77,174 @@ def register_viewset():
 ```
 
 Registering a chooser viewset will also set up a chooser widget to be used whenever a ForeignKey field to that model appears in a `WagtailAdminModelForm` - see [](./forms). In particular, this means that a panel definition such as `FieldPanel("author")`, where `author` is a foreign key to the `Person` model, will automatically use this chooser interface.
+
+## Chooser viewsets for non-model datasources
+
+While the generic chooser views are primarily designed to use Django models as the data source, choosers based on other sources such as REST API endpoints can be implemented by overriding the individual methods that deal with data retrieval.
+
+Within `wagtail.admin.views.generic.chooser`:
+
+-   `BaseChooseView.get_object_list()` - returns a list of records to be displayed in the chooser. (In the default implementation, this is a Django QuerySet, and the records are model instances.)
+-   `BaseChooseView.columns` - a list of `wagtail.admin.ui.tables.Column` objects specifying the fields of the record to display in the final table
+-   `BaseChooseView.apply_object_list_ordering(objects)` - given a list of records as returned from `get_object_list`, returns the list with the desired ordering applied
+-   `ChosenViewMixin.get_object(pk)` - returns the record identified by the given primary key
+-   `ChosenResponseMixin.get_chosen_response_data(item)` - given a record, returns the dictionary of data that will be passed back to the chooser widget to populate it (consisting of items `id` and `title`, unless the chooser widget's JavaScript has been customised)
+
+Within `wagtail.admin.widgets`:
+
+-   `BaseChooser.get_instance(value)` - given a value that may be a record, a primary key or None, returns the corresponding record or None
+-   `BaseChooser.get_value_data_from_instance(item)` - given a record, returns the dictionary of data that will populate the chooser widget (consisting of items `id` and `title`, unless the widget's JavaScript has been customised)
+
+For example, the following code will implement a chooser that runs against a JSON endpoint for the User model at `http://localhost:8000/api/users/`, set up with Django REST Framework using the default configuration and no pagination:
+
+```python
+from django.views.generic.base import View
+import requests
+
+from wagtail.admin.ui.tables import Column, TitleColumn
+from wagtail.admin.views.generic.chooser import (
+    BaseChooseView, ChooseViewMixin, ChooseResultsViewMixin, ChosenResponseMixin, ChosenViewMixin, CreationFormMixin
+)
+from wagtail.admin.viewsets.chooser import ChooserViewSet
+from wagtail.admin.widgets import BaseChooser
+
+
+class BaseUserChooseView(BaseChooseView):
+    @property
+    def columns(self):
+        return [
+            TitleColumn(
+                "title",
+                label="Title",
+                accessor='username',
+                id_accessor='id',
+                url_name=self.chosen_url_name,
+                link_attrs={"data-chooser-modal-choice": True},
+            ),
+            Column(
+                "email", label="Email", accessor="email"
+            )
+        ]
+
+    def get_object_list(self):
+        r = requests.get("http://localhost:8000/api/users/")
+        r.raise_for_status()
+        results = r.json()
+        return results
+
+    def apply_object_list_ordering(self, objects):
+        return objects
+
+
+class UserChooseView(ChooseViewMixin, CreationFormMixin, BaseUserChooseView):
+    pass
+
+
+class UserChooseResultsView(ChooseResultsViewMixin, CreationFormMixin, BaseUserChooseView):
+    pass
+
+
+class UserChosenViewMixin(ChosenViewMixin):
+    def get_object(self, pk):
+        r = requests.get("http://localhost:8000/api/users/%d/" % int(pk))
+        r.raise_for_status()
+        return r.json()
+
+
+class UserChosenResponseMixin(ChosenResponseMixin):
+    def get_chosen_response_data(self, item):
+        return {
+            "id": item["id"],
+            "title": item["username"],
+        }
+
+
+class UserChosenView(UserChosenViewMixin, UserChosenResponseMixin, View):
+    pass
+
+
+class BaseUserChooserWidget(BaseChooser):
+    def get_instance(self, value):
+        if value is None:
+            return None
+        elif isinstance(value, dict):
+            return value
+        else:
+            r = requests.get("http://localhost:8000/api/users/%d/" % int(value))
+            r.raise_for_status()
+            return r.json()
+
+    def get_value_data_from_instance(self, instance):
+        return {
+            "id": instance["id"],
+            "title": instance["username"],
+        }
+
+
+class UserChooserViewSet(ChooserViewSet):
+    icon = "user"
+    choose_one_text = "Choose a user"
+    choose_another_text = "Choose another user"
+    edit_item_text = "Edit this user"
+
+    choose_view_class = UserChooseView
+    choose_results_view_class = UserChooseResultsView
+    chosen_view_class = UserChosenView
+    base_widget_class = BaseUserChooserWidget
+
+
+user_chooser_viewset = UserChooserViewSet("user_chooser", url_prefix="user-chooser")
+```
+
+If the data source implements its own pagination - meaning that the pagination mechanism built into the chooser should be bypassed - the `BaseChooseView.get_results_page(request)` method can be overridden instead of `get_object_list`. This should return an instance of `django.core.paginator.Page`. For example, if the API in the above example followed the conventions of the Wagtail API, implementing pagination with `offset` and `limit` URL parameters and returning a dict consisting of `meta` and `results`, the `BaseUserChooseView` implementation could be modified as follows:
+
+```python
+from django.core.paginator import Page, Paginator
+
+class APIPaginator(Paginator):
+    """
+    Customisation of Django's Paginator class for use when we don't want it to handle
+    slicing on the result set, but still want it to generate the page numbering based
+    on a known result count.
+    """
+    def __init__(self, count, per_page, **kwargs):
+        self._count = int(count)
+        super().__init__([], per_page, **kwargs)
+
+    @property
+    def count(self):
+        return self._count
+
+class BaseUserChooseView(BaseChooseView):
+    @property
+    def columns(self):
+        return [
+            TitleColumn(
+                "title",
+                label="Title",
+                accessor='username',
+                id_accessor='id',
+                url_name=self.chosen_url_name,
+                link_attrs={"data-chooser-modal-choice": True},
+            ),
+            Column(
+                "email", label="Email", accessor="email"
+            )
+        ]
+
+    def get_results_page(self, request):
+        try:
+            page_number = int(request.GET.get('p', 1))
+        except ValueError:
+            page_number = 1
+
+        r = requests.get("http://localhost:8000/api/users/", params={
+            'offset': (page_number - 1) * self.per_page,
+            'limit': self.per_page,
+        })
+        r.raise_for_status()
+        result = r.json()
+        paginator = APIPaginator(result['meta']['total_count'], self.per_page)
+        page = Page(result['items'], page_number, paginator)
+        return page
+```