The value a key (of any field count) indexes and checks uniqueness by. A single-field key uses
the field's own raw value directly - this is the pre-existing, already-shipped index file
format (e.g. a plain "2024-06-02" or 1), and stays exactly as it was for backward compatibility
with real fixture data. A multi-field key has no single value to use, so its fields' values are
combined into one deterministic composite (a JSON array), in the same field order as key.fields
two records only share a composite value if every field matches.
The value a key (of any field count) indexes and checks uniqueness by. A single-field key uses the field's own raw value directly - this is the pre-existing, already-shipped index file format (e.g. a plain "2024-06-02" or 1), and stays exactly as it was for backward compatibility with real fixture data. A multi-field key has no single value to use, so its fields' values are combined into one deterministic composite (a JSON array), in the same field order as key.fields