Allow developers to optimise the queryset used by chooser blocks when rendering

wagtail/blocks/field_block.py

@@ -30,6 +30,9 @@
  from django.forms.fields import CallableChoiceIterator
 
 
+USE_DEFAULT = "__use_default__"
+
+
 class FieldBlock(Block):
  """A block that wraps a Django form field"""
 
@@ -781,22 +784,78 @@
 
 
 class ChooserBlock(FieldBlock):
- def __init__(self, required=True, help_text=None, validators=(), **kwargs):
+ """Abstract superclass for fields that implement a chooser interface (page, image, snippet etc)"""
+
+ # Subclasses can override these attributes to apply 'select_related'
+ # and 'prefetch_related' to querysets when rendering, without having to
+ # supply them explicitly on inititalization each time
+ default_select_related = None
+ default_prefetch_related = None
+
+ def __init__(
+ self,
+ required=True,
+ help_text=None,
+ validators=(),
+ select_related=USE_DEFAULT,
+ prefetch_related=USE_DEFAULT,
+ **kwargs,
+ ):
  self._required = required
  self._help_text = help_text
  self._validators = validators
+ self._select_related = (
+ self.default_select_related
+ if select_related is USE_DEFAULT
+ else select_related
+ )
+ self._prefetch_related = (
+ self.default_prefetch_related
+ if prefetch_related is USE_DEFAULT
+ else prefetch_related
+ )
  super().__init__(**kwargs)
 
- """Abstract superclass for fields that implement a chooser interface (page, image, snippet etc)"""
+ def deconstruct(self):
+ """
+ Overrides ``FieldBlock.decontruct()`` to exclude 'select_related' and
+ 'prefetch_related' values provided on initialisation, as they are considered
+ 'rendering optimisations', that do not impact stored data or validation.
+ """
+ name, args, kwargs = super().deconstruct()
+ try:
+ del kwargs["select_related"]
+ except KeyError:
+ pass
+ try:
+ del kwargs["prefetch_related"]
+ except KeyError:
+ pass
+ return name, args, kwargs
 
  @cached_property
  def model_class(self):
  return resolve_model_string(self.target_model)
 
+ def get_base_queryset(self):
+ return self.model_class.objects.all()
+
+ def get_display_queryset(self):
+ """
+ Override this method to apply any additional filtering to the queryset
+ for this block when converting raw values into full objects for display.
+ """
+ qs = self.get_base_queryset()
+ if self._select_related is not None:
+ qs = qs.select_related(self._select_related)
+ if self._prefetch_related is not None:
+ qs = qs.prefetch_related(self._prefetch_related)
+ return qs
+
  @cached_property
  def field(self):
  return forms.ModelChoiceField(
- queryset=self.model_class.objects.all(),
+ queryset=self.get_base_queryset(),
  widget=self.widget,
  required=self._required,
  validators=self._validators,
@@ -809,7 +868,7 @@
  return value
  else:
  try:
- return self.model_class.objects.get(pk=value)
+ return self.get_display_queryset().get(pk=value)
  except self.model_class.DoesNotExist:
  return None
 
@@ -818,7 +877,7 @@
 
  The instances must be returned in the same order as the values and keep None values.
  """
- objects = self.model_class.objects.in_bulk(values)
+ objects = self.get_display_queryset().in_bulk(values)
  return [
  objects.get(id) for id in values
  ] # Keeps the ordering the same as in values.
@@ -836,7 +895,7 @@
  return value
  else:
  try:
- return self.model_class.objects.get(pk=value)
+ return self.get_base_queryset().get(pk=value)
  except self.model_class.DoesNotExist:
  return None