For instance, the following class definition will create an index for its SerialNumber property (up to 30 characters long if stored in an external database), and will expect a link to a model of diaper (TSQLDiaperModel) and the baby which used it (TSQLBaby).
An ID / RowID column will always be available (from TSQLRecord), so in this case, you would be able to make a fast look-up for a particular diaper from either its internal mORmot ID, or its official unique serial number:

/// table used for the Diaper queries
TSQLDiaper = class(TSQLRecord)
    fSerialNumber: RawUTF8;
    fModel: TSQLDiaperModel;
    fBaby: TSQLBaby;
    property SerialNumber: RawUTF8 index 30 read fSerialNumber write fSerialNumber
     stored AS_UNIQUE;
    property Model: TSQLDiaperModel read fModel write fModel;
    property Baby: TSQLBaby read fBaby write fBaby;

Just a constant definition, and code sounds more readable.
It is worth saying that Delphi SOAP units uses a similar trick, by defining stored AS_ATTRIBUTE in its TRemotable classes.