Function foreign_key_values 

pub fn foreign_key_values(
    table: &str,
    fk: &ForeignKey,
    document: &ResultRow,
    comparison: &ForeignKeyComparison,
) -> Result<Option<Vec<Value>>, SQLError>

The normalized values of the foreign key that the row of table holds, or None when NULLs exempt the row: any NULL under MATCH SIMPLE, all of them under MATCH FULL, which rejects a key that mixes NULLs with values as RI_FKey_check does.