Add server-side min_num and max_num validation for ListBlock

Fixes #5300

docs/reference/streamfield/blocks.rst

@@ -440,9 +440,11 @@ Structural block types
            ]))),
        ])
 
-   The following additional option is available as either a keyword argument or a Meta class attribute:
+   The following additional options are available as either keyword arguments or Meta class attributes:
 
    :param form_classname: An HTML ``class`` attribute to set on the root element of this block as displayed in the editing interface.
+   :param min_num: Minimum number of sub-blocks that the list must have.
+   :param max_num: Maximum number of sub-blocks that the list may have.
 
 
 .. class:: wagtail.core.blocks.StreamBlock

wagtail/core/blocks/list_block.py

@@ -98,6 +98,16 @@ class ListBlock(Block):
             else:
                 errors.append(None)
 
+        if self.meta.min_num is not None and self.meta.min_num > len(value):
+            non_block_errors.append(ValidationError(
+                _('The minimum number of items is %d') % self.meta.min_num
+            ))
+
+        if self.meta.max_num is not None and self.meta.max_num < len(value):
+            non_block_errors.append(ValidationError(
+                _('The maximum number of items is %d') % self.meta.max_num
+            ))
+
         if any(errors) or non_block_errors:
             raise ListBlockValidationError(block_errors=errors, non_block_errors=non_block_errors)
 
@@ -172,6 +182,10 @@ class ListBlock(Block):
         # descendant block type
         icon = "placeholder"
         form_classname = None
+        min_num = None
+        max_num = None
+
+    MUTABLE_META_ATTRIBUTES = ['min_num', 'max_num']
 
 
 class ListBlockAdapter(Adapter):
@@ -193,6 +207,12 @@ class ListBlockAdapter(Adapter):
             meta['helpText'] = help_text
             meta['helpIcon'] = get_help_icon()
 
+        if block.meta.min_num is not None:
+            meta['minNum'] = block.meta.min_num
+
+        if block.meta.max_num is not None:
+            meta['maxNum'] = block.meta.max_num
+
         return [
             block.name,
             block.child_block,

wagtail/core/tests/test_blocks.py

@@ -2179,6 +2179,34 @@ class TestListBlock(WagtailTestUtils, SimpleTestCase):
             },
         })
 
+    def test_adapt_with_min_num_max_num(self):
+        class LinkBlock(blocks.StructBlock):
+            title = blocks.CharBlock()
+            link = blocks.URLBlock()
+
+        block = blocks.ListBlock(LinkBlock, min_num=2, max_num=5)
+
+        block.set_name('test_listblock')
+        js_args = ListBlockAdapter().js_args(block)
+
+        self.assertEqual(js_args[0], 'test_listblock')
+        self.assertIsInstance(js_args[1], LinkBlock)
+        self.assertEqual(js_args[2], {'title': None, 'link': None})
+        self.assertEqual(js_args[3], {
+            'label': 'Test listblock',
+            'icon': 'placeholder',
+            'classname': None,
+            'minNum': 2,
+            'maxNum': 5,
+            'strings': {
+                'DELETE': 'Delete',
+                'DUPLICATE': 'Duplicate',
+                'MOVE_DOWN': 'Move down',
+                'MOVE_UP': 'Move up',
+                'ADD': 'Add',
+            },
+        })
+
     def test_searchable_content(self):
         class LinkBlock(blocks.StructBlock):
             title = blocks.CharBlock()
@@ -2332,6 +2360,32 @@ class TestListBlock(WagtailTestUtils, SimpleTestCase):
             },
         })
 
+    def test_min_num_validation_errors(self):
+        block = blocks.ListBlock(blocks.CharBlock(), min_num=2)
+
+        with self.assertRaises(ValidationError) as catcher:
+            block.clean(['foo'])
+        self.assertEqual(catcher.exception.params, {
+            'block_errors': [None],
+            'non_block_errors': ['The minimum number of items is 2']
+        })
+
+        # a value with >= 2 blocks should pass validation
+        self.assertTrue(block.clean(['foo', 'bar']))
+
+    def test_max_num_validation_errors(self):
+        block = blocks.ListBlock(blocks.CharBlock(), max_num=2)
+
+        with self.assertRaises(ValidationError) as catcher:
+            block.clean(['foo', 'bar', 'baz'])
+        self.assertEqual(catcher.exception.params, {
+            'block_errors': [None, None, None],
+            'non_block_errors': ['The maximum number of items is 2']
+        })
+
+        # a value with <= 2 blocks should pass validation
+        self.assertTrue(block.clean(['foo', 'bar']))
+
 
 class TestListBlockWithFixtures(TestCase):
     fixtures = ['test.json']