Add validations for min_max_fields on StreamBlock

docs/topics/streamfield.rst

@@ -441,7 +441,7 @@ Since ``StreamField`` accepts an instance of ``StreamBlock`` as a parameter, in
 .. code-block:: python
 
     class HomePage(Page):
-        carousel = StreamField(CarouselBlock(max_num=10))
+        carousel = StreamField(CarouselBlock(max_num=10, min_max_fields={'video': {'max_num': 2}}))
 
 ``StreamBlock`` accepts the following options as either keyword arguments or ``Meta`` properties:
 
@@ -454,6 +454,9 @@ Since ``StreamField`` accepts an instance of ``StreamBlock`` as a parameter, in
 ``max_num``
   Maximum number of sub-blocks that the stream may have.
 
+``min_max_fields``
+  Specifies the minimum and maximum number of each block type, as a dictionary mapping block names to dicts with (optional) ``min_num`` and ``max_num`` fields.
+
 
 .. _streamfield_personblock_example:
 

wagtail/wagtailcore/blocks/stream_block.py

@@ -36,12 +36,15 @@ class StreamBlockValidationError(ValidationError):
 
 class BaseStreamBlock(Block):
 
-    def __init__(self, local_blocks=None, min_num=None, max_num=None, **kwargs):
+    def __init__(self, local_blocks=None, min_num=None, max_num=None, min_max_fields=None, **kwargs):
         self._constructor_kwargs = kwargs
 
         # Used to validate the minimum and maximum number of elements in the block
         self.min_num = min_num
         self.max_num = max_num
+        if min_max_fields is None:
+            min_max_fields = {}
+        self.min_max_fields = min_max_fields
 
         super(BaseStreamBlock, self).__init__(**kwargs)
 
@@ -212,6 +215,33 @@ class BaseStreamBlock(Block):
                 [_('The maximum number of items is %s' % self.max_num)]
             ))
 
+        if self.min_max_fields:
+            fields = {}
+            for item in value:
+                if item.block_type not in self.min_max_fields:
+                    continue
+                if item.block_type not in fields:
+                    fields[item.block_type] = 0
+                fields[item.block_type] += 1
+
+            for field in self.min_max_fields:
+                field_title = field.replace('_', ' ').title()
+                max_num = self.min_max_fields[field].get('max_num', None)
+                min_num = self.min_max_fields[field].get('min_num', None)
+                if field in fields:
+                    if min_num and min_num > fields[field]:
+                        non_block_errors.append(ErrorList(
+                            ['{}: {}'.format(field_title, _('The minimum number of items is %s' % min_num))]
+                        ))
+                    if max_num and max_num < fields[field]:
+                        non_block_errors.append(ErrorList(
+                            ['{}: {}'.format(field_title, _('The maximum number of items is %s' % max_num))]
+                        ))
+                elif min_num:
+                        non_block_errors.append(ErrorList(
+                            ['{}: {}'.format(field_title, _('The minimum number of items is %s' % min_num))]
+                        ))
+
         if errors or non_block_errors:
             # The message here is arbitrary - outputting error messages is delegated to the child blocks,
             # which only involves the 'params' list

wagtail/wagtailcore/tests/test_blocks.py

@@ -2073,6 +2073,42 @@ class TestStreamBlock(WagtailTestUtils, SimpleTestCase):
             '__all__': [['The maximum number of items is 1']]
         })
 
+    def test_min_max_fields_min_validation_errors(self):
+        class ValidatedBlock(blocks.StreamBlock):
+            char = blocks.CharBlock()
+            url = blocks.URLBlock()
+        block = ValidatedBlock(min_max_fields={'char': {'min_num': 1}})
+
+        value = blocks.StreamValue(block, [
+            ('url', 'http://example.com/'),
+            ('url', 'http://example.com/'),
+        ])
+
+        with self.assertRaises(ValidationError) as catcher:
+            block.clean(value)
+        self.assertEqual(catcher.exception.params, {
+            '__all__': [['Char: The minimum number of items is 1']]
+        })
+
+    def test_min_max_fields_max_validation_errors(self):
+        class ValidatedBlock(blocks.StreamBlock):
+            char = blocks.CharBlock()
+            url = blocks.URLBlock()
+        block = ValidatedBlock(min_max_fields={'char': {'max_num': 1}})
+
+        value = blocks.StreamValue(block, [
+            ('char', 'foo'),
+            ('char', 'foo'),
+            ('url', 'http://example.com/'),
+            ('url', 'http://example.com/'),
+        ])
+
+        with self.assertRaises(ValidationError) as catcher:
+            block.clean(value)
+        self.assertEqual(catcher.exception.params, {
+            '__all__': [['Char: The maximum number of items is 1']]
+        })
+
     def test_block_level_validation_renders_errors(self):
         block = FooStreamBlock()