Missing keys or values [_missing_keys]

When you access document values, using doc['myfield'].value throws an exception if the specified field is missing in a document.

For more dynamic index mappings, you can write a catch equation:

if (!doc.containsKey('myfield') || doc['myfield'].empty) { return "unavailable" } else { return doc['myfield'].value }

This expression tests for the existence of myfield, returning its value only if the key exists.

To check if a document is missing a value, call doc['myfield'].size() == 0.