asymmetric XREFs

In this section we discuss the consequences of providing data to an asymmetric XREF. Assume the following structure:

 OWNER     ?TShirt Borrowed_by ?Person XREF TShirt


 TARGET    ?Person  TShirt ?TShirt
Activate the XREF with the following data:

TShirt : sam15
Borrowed_by Sarah
As per the XREF rules, ACEDB looks for Sarah in the Person class and fills in "sam15" following the "TShirt" label. Remember that if Sarah does not yet exist, ACEDB will create a Person with this name. Reusing the diagram from the previous section:


          In Sam15, enter data "Sarah"--------
         into field ?Person XREF TShirt      |
                                             |
                                             |
                                             |
 OWNER     -----?TShirt Borrowed_by ?Person XREF TShirt
 Sam15     |                           |           |
           |                           |           |
           |            1. Find        |           |
           |       ---------------------           |
           |       |   Person Sarah                |
           |       |                               |
           |       |                               |
           |       |               2. Find         |
           |       |      --------------------------
           |       |      |      label TShirt
           |       |      |     in Person Sarah
           |       |      |
           |       |      |
 TARGET    |    ?Person TShirt ?TShirt
 Sarah     |                       |
           |                       |
           |                       |
           |       3. Message:     |
           -------------------------
                 Put TShirt Sam15
                into ?TShirt field
                  in Person Sarah
Because XREF automatically fills out the ?TShirt field in Person Sarah, it is as if this data had been entered in the first place:

TShirt : sam15
Borrowed_by Sarah
 
Person : Sarah
TShirt sam15 //**Gratis!!
The major advantage of XREF is that it fills out the other side of the two-way link--meaning the curator doesn't have to. With XREF there is no risk of intending to establish a two-way link and then forgetting to fill out both sides; this is always a danger without XREF.

Consequences of Asymmetry

The asymmetry of our XREF link means that entering data on the owner side of a link, in contrast with the target side, has different effects. Consider what happens if data is entered directly into a Person object:

Person : Sarah
TShirt sam55
In this case the software creates a TShirt class member called "sam55" (if it does not already exist)--but that is all. The link from ?Person to ?TShirt is ordinary; there is no XREF to fill out a field in ?TShirt automatically. Specifically, this field in ?TShirt:

Borrowed_by ?Person XREF TShirt is not filled out, and this means that a one-way link has been established, not a two-way link. Thus

an XREF is required for an object to send its name to a target.
It is possible to establish a symmetrical relationship by using XREFs on both sides of a link. This is considered in the next section.

Show me the whole t-shirt model again.

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