Release 10.1A: OpenEdge Development:
ProDataSets

Using the REPOSITION Data-Relation

You can include the optional keyword REPOSITION on a Data-Relation definition that is part of a ProDataSet definition. As you will later see, there is also a REPOSITION logical attribute on a Data-Relation handle that lets you set this mode on or off at run time.

The purpose of the reposition mode on a relation is to handle the situation where you want to load a limited set of child records into a ProDataSet table that is related to possibly multiple parent rows of another table in the ProDataSet. For example, you might want to include a separate State table with state abbreviation and full state name in your ProDataSet rather than joining that information into each individual Customer or Order record in the ProDataSet. Or you might want to include an Item table with all available Items in your ProDataSet and be able to join an Item to all of the OrderLines that include that Item.

If REPOSITION mode is true at the time of a FILL, either because the keyword is part of the definition or because you have set the attribute to true, then Progress ignores the Data-Relation that would otherwise filter the children to retrieve only those related to a parent of the relation, and fills the table as a top-level table.

Example with REPOSITION not set

For example, consider an Order ProDataSet with a Data-Relation from ttOrderLine to ttItem, with relation fields of ttOrderline.ItemNum and ttItem.ItemNum. If you do not define or set the REPOSITION attribute, then Progress loads into ttItem the Item information for every Item used in at least one OrderLine that is also loaded into the ProDataSet. There might well be multiple OrderLines that use the same Order, but because the default fill processing is to discard records with duplicate keys, Progress automatically discards all duplicates based on the unique index definition for the ttItem temp-table, and you wind up with exactly one instance of each Item used on the ProDataSet’s OrderLines. If there is no unique index on the ttItem temp-table, then you wind up with one instance of ttItem for each OrderLine that uses a given Item, which is almost certainly not what you want.

Example with REPOSITION set: loading

Alternatively, if you define or set the REPOSITION attribute of the relation, when Progress fills the ProDataSet, it disregards the relation fields of that Data-Relation and fills that table independent of its parent. This means that by default Progress loads all records from the child table’s database buffer into the temp-table. Or if you have prepared a specific query for the child table’s Data-Source, then the records that satisfy the query are loaded.

Using the same example for OrderLines and their Items, with REPOSITION set to true, Progress loads all Items into ttItem regardless of whether they are used by OrderLines in this instance of the ProDataSet. This makes the entire list of Items available for display and selection in the user interface, or within business logic that uses the ProDataSet. You can set REPOSITION true or false for this type of relation, depending on your application requirements.

Example with REPOSITION set: navigation

During navigation, REPOSITION mode serves a somewhat different purpose. As you learned in the introduction, Progress generates a dynamic query for each child table of a Data-Relation that you can use to browse the current records at that level, where the meaning of “current” is normally the set of children for the currently selected parent. If REPOSITION is true when your application is navigating the ProDataSet, then the query works differently. The query does not filter children, but selects all rows in the child table regardless of the parent. However, the child query is repositioned to the child row for the currently selected parent.

Once again using the same example, if your user interface uses browse objects to show OrderLines of the current Order in one browse and Items in another browse, then with REPOSITION False, the Item browse will show only the one ttItem row for the current ttOrderLine. With REPOSITION True, the ttItem browse displays all Items and repositions to the Item for the current OrderLine. This REPOSITION mode has no other effect when you are navigating the ProDataSet. Again, set REPOSITION to True or False for this kind of many-to-one relationship, depending on which default behavior you want.