I've found a problem with the treatment of missing values in feeds. Steps to re-create:
- Create a data source in a feed
- Use a Filter operator to filter the feed until there are no rows returned
- Use a Feed to Value operator to convert to a single value
A preview of the output shows the Single Value is missing/blank/null (the UI doesn't make it clear). Now when you attach that value to another operator you get inconsistent results:
- Attaching this as a condition in a Filter operator, the value is treated as an empty string and will filter using that value
- Attaching this as a condition to a Replace Values operator, the value is treated as missing, and the condition will default according to the option you select (met, not met)
- Create a User Input operator in the feed
- Do not assign a user input to it in the display
Now when you attach this value as a condition to the Filter or Replace Values operator, they work in the same way, as if the value is missing.
What this means is that you can't have a missing value generated by a feed, only by user input, and even then, only by using the display elements "no selection" or "delete selection" options.
Is there any other way to generate a missing value within a feed to get round these problems?