Add ability to specify min/max block counts on StreamBlock

docs/topics/streamfield.rst

@@ -441,9 +441,18 @@ Since ``StreamField`` accepts an instance of ``StreamBlock`` as a parameter, in
 .. code-block:: python
 
     class HomePage(Page):
-        carousel = StreamField(CarouselBlock())
+        carousel = StreamField(CarouselBlock(max_num=10))
 
-``StreamBlock`` accepts ``required`` as a keyword argument or ``Meta`` property; if true (the default), at least one sub-block must be supplied.
+``StreamBlock`` accepts the following options as either keyword arguments or ``Meta`` properties:
+
+``required`` (default: True)
+  If true, at least one sub-block must be supplied.
+
+``min_num``
+  Minimum number of sub-blocks that the stream must have.
+
+``max_num``
+  Maximum number of sub-blocks that the stream may have.
 
 
 .. _streamfield_personblock_example:

wagtail/wagtailcore/blocks/stream_block.py

@@ -13,6 +13,7 @@ from django.utils import six
 from django.utils.encoding import python_2_unicode_compatible
 from django.utils.html import format_html_join
 from django.utils.safestring import mark_safe
+from django.utils.translation import ugettext as _
 
 from wagtail.wagtailcore.utils import escape_script
 
@@ -35,9 +36,13 @@ class StreamBlockValidationError(ValidationError):
 
 class BaseStreamBlock(Block):
 
-    def __init__(self, local_blocks=None, **kwargs):
+    def __init__(self, local_blocks=None, min_num=None, max_num=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
+
         super(BaseStreamBlock, self).__init__(**kwargs)
 
         # create a local (shallow) copy of base_blocks so that it can be supplemented by local_blocks
@@ -196,6 +201,17 @@ class BaseStreamBlock(Block):
         if self.required and len(value) == 0:
             non_block_errors.append(ValidationError('This field is required', code='invalid'))
 
+        # Validate that the min_num and max_num has a value
+        # and if it does meet the conditions of the number of components in the block
+        if self.min_num and self.min_num > len(value):
+            non_block_errors.append(ErrorList(
+                [_('The minimum number of items is %s' % self.min_num)]
+            ))
+        if self.max_num and self.max_num < len(value):
+            non_block_errors.append(ErrorList(
+                [_('The maximum number of items is %s' % self.max_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

@@ -2040,6 +2040,39 @@ class TestStreamBlock(WagtailTestUtils, SimpleTestCase):
             3: ['Enter a valid URL.'],
         })
 
+    def test_min_num_validation_errors(self):
+        class ValidatedBlock(blocks.StreamBlock):
+            char = blocks.CharBlock()
+            url = blocks.URLBlock()
+        block = ValidatedBlock(min_num=1)
+
+        value = blocks.StreamValue(block, [])
+
+        with self.assertRaises(ValidationError) as catcher:
+            block.clean(value)
+        self.assertEqual(catcher.exception.params, {
+            '__all__': [['The minimum number of items is 1']]
+        })
+
+    def test_max_num_validation_errors(self):
+        class ValidatedBlock(blocks.StreamBlock):
+            char = blocks.CharBlock()
+            url = blocks.URLBlock()
+        block = ValidatedBlock(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__': [['The maximum number of items is 1']]
+        })
+
     def test_block_level_validation_renders_errors(self):
         block = FooStreamBlock()