Describe a relationship once, then let a consumer navigate it

Orders refer to carriers by code. A CDS association models that relationship in the view definition and can be exposed for consumers to navigate when needed.

association [0..1] to zcarrier as _Carrier
  on $projection.CarrierCode = _Carrier.carrier_code

The cardinality [0..1] says there may be no carrier or one carrier for an order. It must match actual data and business constraints. Declaring [0..1] when multiple carrier rows can match is not a harmless optimization; it misrepresents the relationship.

Expose the association in the projection when consumers should be able to navigate it:

_Carrier

Association or explicit join?

A join combines source fields in one query result. An association names a reusable relationship and supports path navigation in CDS/ABAP SQL. Associations may be instantiated as joins when consumed; they are not simply preloaded child objects. Use an explicit join for a one-off combination; use an association when the relationship is part of the reusable model.

Practice

Add the association, ON condition, and projection exposure. This DDL exercise checks fragments only; no CDS activation occurs here.

Sources

• SAP Samples, CDS View Entities: associations and joins
• SAP Help Portal, ABAP Core Data Services

Version note: CDS association features and cardinality semantics depend on ABAP release; validate against the target feature table.

define view entity ZI_DISPATCH_WITH_CARRIER
  as select from zdispatch_order as Order
  association [0..1] to zcarrier as _Carrier
    on $projection.CarrierCode = _Carrier.carrier_code
{
  key Order.dispatch_order as DispatchOrder,
      Order.carrier_code   as CarrierCode,
      _Carrier
}