Add bulk_to_python method to the base Block class

wagtail/core/blocks/base.py

@@ -213,6 +213,14 @@ class Block(metaclass=BaseBlock):
         """
         return value
 
+    def bulk_to_python(self, values):
+        """
+        Apply the to_python conversion to a list of values. The default implementation simply
+        iterates over the list; subclasses may optimise this, e.g. by combining database lookups
+        into a single query.
+        """
+        return [self.to_python(value) for value in values]
+
     def get_prep_value(self, value):
         """
         The reverse of to_python; convert the python value into JSON-serialisable form.

wagtail/core/blocks/list_block.py

@@ -134,15 +134,8 @@ class ListBlock(Block):
         return result
 
     def to_python(self, value):
-        # If child block supports bulk retrieval, use it.
+        # 'value' is a list of child block values; use bulk_to_python to convert them all in one go
-        if hasattr(self.child_block, 'bulk_to_python'):
+        return self.child_block.bulk_to_python(value)
-            return self.child_block.bulk_to_python(value)
-
-        # Otherwise recursively call to_python on each child and return as a list.
-        return [
-            self.child_block.to_python(item)
-            for item in value
-        ]
 
     def get_prep_value(self, value):
         # recursively call get_prep_value on children and return as a list

wagtail/core/blocks/stream_block.py

@@ -390,12 +390,8 @@ class StreamValue(Sequence):
                 raw_value = self.stream_data[i]
                 type_name = raw_value['type']
                 child_block = self.stream_block.child_blocks[type_name]
-                if hasattr(child_block, 'bulk_to_python'):
+                self._prefetch_blocks(type_name, child_block)
-                    self._prefetch_blocks(type_name, child_block)
+                return self._bound_blocks[i]
-                    return self._bound_blocks[i]
-                else:
-                    value = child_block.to_python(raw_value['value'])
-                    block_id = raw_value.get('id')
             else:
                 try:
                     type_name, value, block_id = self.stream_data[i]