implement binary field size validation

mongoengine/fields.py

@@ -448,7 +448,8 @@ class BinaryField(BaseField):
     """A binary data field.
     """
 
-    def __init__(self, **kwargs):
+    def __init__(self, max_bytes=None, **kwargs):
+        self.max_bytes = max_bytes
         super(BinaryField, self).__init__(**kwargs)
 
     def to_mongo(self, value):
@@ -456,3 +457,9 @@ class BinaryField(BaseField):
 
     def to_python(self, value):
         return str(value)
+
+    def validate(self, value):
+        assert isinstance(value, str)
+
+        if self.max_bytes is not None and len(value) > self.max_bytes:
+            raise ValidationError('Binary value is too long')

tests/fields.py

@@ -516,5 +516,40 @@ class FieldTest(unittest.TestCase):
 
         Attachment.drop_collection()
 
+    def test_binary_validation(self):
+        """Ensure that invalid values cannot be assigned to binary fields.
+        """
+        class Attachment(Document):
+            blob = BinaryField()
+
+        class AttachmentRequired(Document):
+            blob = BinaryField(required=True)
+
+        class AttachmentSizeLimit(Document):
+            blob = BinaryField(max_bytes=4)
+
+        Attachment.drop_collection()
+        AttachmentRequired.drop_collection()
+        AttachmentSizeLimit.drop_collection()
+
+        attachment = Attachment()
+        attachment.validate()
+        attachment.blob = 2
+        self.assertRaises(ValidationError, attachment.validate)
+
+        attachment_required = AttachmentRequired()
+        self.assertRaises(ValidationError, attachment_required.validate)
+        attachment_required.blob = '\xe6\x00\xc4\xff\x07'
+        attachment_required.validate()
+
+        attachment_size_limit = AttachmentSizeLimit(blob='\xe6\x00\xc4\xff\x07')
+        self.assertRaises(ValidationError, attachment_size_limit.validate)
+        attachment_size_limit.blob = '\xe6\x00\xc4\xff'
+        attachment_size_limit.validate()
+
+        Attachment.drop_collection()
+        AttachmentRequired.drop_collection()
+        AttachmentSizeLimit.drop_collection()
+
 if __name__ == '__main__':
     unittest.main()