Understanding Your Data Model › Using Relations › Example of Using Extended by Relations

Example of Using Extended by Relations

Your basic Customer information consists of the following fields:

FIL         Customer        REF     Known by        FLD     Customer code           CDE

FIL  Customer        REF     Has             FLD     Customer name           TXT

FIL  Customer        REF     Extended by     FIL     Customer detail         REF

You may enter additional details about Customers in another file called Customer detail:

FIL  Customer detail REF     Owned by        FIL     Customer                CDE

FIL  Customer detail REF     Has             FLD     Credit limit    VAL

FIL  Customer detail REF     Has             FLD     Managing Director       TXT

This results in the following entries:

If you wish to create a function that brings both Customer and Customer detail together, you need to build an access path that contains fields from both files. If you were doing this on the Customer detail file, this would not present a problem since you could specify virtual fields on the Owned by relation

FIL    Customer detail REF     Owned by        FIL     Customer                CDE

     VRT     Customer name   TXT

FIL  Customer detail REF     Has             FLD     Credit limit    VAL

FIL  Customer detail REF     Has             FLD     Managing Director       TXT

However, if you wish to attach your function to the Customer file, you would not be able to obtain the customer details unless you had an Extended by relation. Using the Extended by relation, you can specify virtual fields as follows:

FIL        Customer detail REF     Known by        FLD     Customer code           CDE

FIL  Customer detail REF     Has             FLD     Customer name   TXT

FIL  Customer detail REF     Extended by     FIL     Customer detail REF

                                             VRT     Credit limit    VAL

                                             VRT     Managing Director       TXT

Each Customer may have only one Customer detail record: there is a one-to-one correspondence between files.

There are two implementation reasons why you may consider using the Extended by relation rather than simply including the data from the extended file in the basic file:

• To save space. If some data fields are only present on a minority of records, then it may be desirable to place the rarely used fields into an Extended by file.
• To avoid recompilation of an existing system. If you wish to add fields to an existing file that is already used by a large number of programs, you could avoid level check problems by placing the extra fields in another file owned by the original file. An Extended by relation would make the new file details available from the based-on file.

  Note: An Extended by relation effectively constrains an Owned by relation, which is normally one-to-many, to be a one-to-one relationship. It is only appropriate to use the Extended by relation for cases where a one-to-one or one-to-none relationship holds.