Lesson 5 — CDS Associations & Path Expressions

In Lesson 4, you combined related data with explicit joins:

/DMO/CONNECTION
   ├─ INNER JOIN Carrier
   ├─ INNER JOIN Origin Airport
   └─ INNER JOIN Destination Airport
            ↓
        Route model

That solved an immediate problem: combine several sources into one result.

The relationships themselves are useful beyond one fixed result:

Connection → Carrier
Connection → Origin Airport
Connection → Destination Airport

ABAP CDS provides associations to model those relationships explicitly and make them reusable.

In this lesson, you will build:

/DMO/CONNECTION
      ↓
ZI_ROUTE_ASSOC_###
      ├── _Carrier ─────────────► /DMO/I_CARRIER
      ├── _OriginAirport ───────► /DMO/I_AIRPORT
      └── _DestinationAirport ──► /DMO/I_AIRPORT

You will use path expressions to retrieve target fields, expose the associations themselves, and follow those associations directly in ADT Data Preview.

---

Lesson information

Duration	About 120–150 minutes
Environment	SAP BTP ABAP Trial
Development tool	Eclipse with ABAP Development Tools (ADT)
Prerequisites	Lesson 1, Lesson 2, Lesson 3, and Lesson 4
Primary source	/DMO/CONNECTION
Association targets	/DMO/I_CARRIER, /DMO/I_AIRPORT
CDS entity	ZI_ROUTE_ASSOC_###
Reader	ZCL_ROUTE_ASSOC_READER_###
Runner	ZCL_CDS_L05_###

---

Learning objectives

After completing this lesson, you should be able to:

• explain what a CDS association represents;
• distinguish an explicit join from an association;
• explain that associations are directional;
• define a CDS association;
• identify its source and target;
• choose a meaningful association alias;
• explain [0..1], [1..1], [0..*], and [1..*];
• choose cardinality based on the real relationship;
• write an association ON condition;
• use $projection;
• explain why a to-one association should normally match the complete target key;
• use CDS path expressions;
• explain what happens when a path target field is used;
• expose an association in the element list;
• follow an exposed association in ADT Data Preview;
• distinguish a scalar path field from an exposed relationship;
• query scalar association-derived fields in SQL Console;
• consume the CDS model through a Reader;
• explain how associations prepare the model for RAP and service navigation.

---

Before you start

Replace ### with your student/group suffix.

Example:

ZI_ROUTE_ASSOC_###         → ZI_ROUTE_ASSOC_017
ZCL_ROUTE_ASSOC_READER_### → ZCL_ROUTE_ASSOC_READER_017
ZCL_CDS_L05_###            → ZCL_CDS_L05_017

ABAP Cloud and /DMO/*

The /DMO/* artifacts used here are SAP Flight Reference Scenario demo/tutorial content.

For productive ABAP Cloud development, consume SAP functionality through APIs released for the applicable cloud-development contract.

---

1. Revisit the Lesson 4 model

Lesson 4 used explicit joins:

inner join /dmo/carrier as Carrier
  on Connection.carrier_id = Carrier.carrier_id

inner join /dmo/airport as OriginAirport
  on Connection.airport_from_id = OriginAirport.airport_id

inner join /dmo/airport as DestinationAirport
  on Connection.airport_to_id = DestinationAirport.airport_id

This says:

For this CDS result, combine these sources now.

Associations let us say something more reusable:

A Connection has a Carrier relationship.
A Connection has an Origin Airport relationship.
A Connection has a Destination Airport relationship.

---

2. What is a CDS association?

A CDS association is a named relationship from an association source to an association target.

Conceptually:

Connection ── _Carrier ──► Carrier

An association has:

target
name/alias
cardinality
ON condition

The ON condition describes how source and target records correspond.

TIP

Think of an association as a named navigation path between data entities.

---

3. Associations are directional

If you define:

Connection → Carrier

that does not automatically define:

Carrier → Connection

The association belongs to its source entity.

If reverse navigation is needed, it needs its own model relationship.

This becomes important later when RAP models parent/child relationships explicitly.

---

4. Why this lesson uses CDS targets

This lesson targets SAP's demo CDS entities:

/DMO/I_CARRIER
/DMO/I_AIRPORT

rather than directly targeting the database tables.

The Flight Reference Scenario exposes:

/DMO/I_CARRIER
  AirlineID
  Name
  CurrencyCode

and:

/DMO/I_AIRPORT
  AirportID
  Name
  City
  CountryCode

This gives a useful architecture:

Your CDS entity
      ↓ association
Reusable target CDS entity
      ↓
Persistence

---

5. Inspect /DMO/I_CARRIER

Press:

Ctrl + Shift + A

Search for:

/DMO/I_CARRIER

Open its Data Definition.

Identify:

AirlineID
Name
CurrencyCode

The target key is:

AirlineID

Use Data Preview if useful.

---

6. Inspect /DMO/I_AIRPORT

Open:

/DMO/I_AIRPORT

Identify:

AirportID
Name
City
CountryCode

The target key is:

AirportID

Before writing any association, you should understand the key of its target.

---

7. Association syntax

A basic association looks like:

association [1..1] to /DMO/I_Carrier as _Carrier
  on <condition>

Break it down:

Part	Meaning
association	define a relationship
[1..1]	target cardinality
to	target follows
/DMO/I_Carrier	target CDS entity
_Carrier	association name
on	relationship condition

---

8. Association naming

This course uses names such as:

_Carrier
_OriginAirport
_DestinationAirport

The leading underscore is a common SAP CDS naming style for associations.

The underscore is not a CDS keyword and is not what makes something an association.

Use names that describe the target's role.

---

9. Cardinality

Cardinality describes how many target records are expected for one source record.

Common forms:

Cardinality	Meaning
[0..1]	zero or one
[1..1]	exactly one
[0..*]	zero, one, or many
[1..*]	one or many

For the demo connection model:

one Connection → one Carrier
one Connection → one Origin Airport
one Connection → one Destination Airport

Use:

[1..1]

for these relationships.

Cardinality is part of the model

Do not copy [1..1] mechanically.

Cardinality must agree with the real relationship and the ON condition.

---

10. Create ZI_ROUTE_ASSOC_###

Right-click your package:

New
  → Other ABAP Repository Object
  → Core Data Services
  → Data Definition

Enter:

Name:        ZI_ROUTE_ASSOC_###
Description: Lesson 5 - Route Associations

Choose the course transport if required.

Select a CDS View Entity template.

Confirm:

define view entity

and finish.

---

11. Start with the Connection fields

Enter:

@AccessControl.authorizationCheck: #NOT_REQUIRED
@EndUserText.label: 'Route with Associations'

define view entity ZI_Route_Assoc_###
  as select from /dmo/connection as Connection
{
  key Connection.carrier_id       as CarrierId,
  key Connection.connection_id    as ConnectionId,

      Connection.airport_from_id  as OriginAirportId,
      Connection.airport_to_id    as DestinationAirportId,
      Connection.departure_time   as DepartureTime,
      Connection.arrival_time     as ArrivalTime,
      Connection.distance         as Distance,
      Connection.distance_unit    as DistanceUnit
}

Replace ###.

Check:

Ctrl + F2

Activate:

Ctrl + F3

Preview:

F8

---

12. Add _Carrier

Place the association between the data source and the element list:

association [1..1] to /DMO/I_Carrier as _Carrier
  on $projection.CarrierId = _Carrier.AirlineID

Your source section becomes:

define view entity ZI_Route_Assoc_###
  as select from /dmo/connection as Connection

    association [1..1] to /DMO/I_Carrier as _Carrier
      on $projection.CarrierId = _Carrier.AirlineID

{
  ...
}

---

13. What is $projection?

The persistence source provides:

Connection.carrier_id

Your CDS entity exposes it as:

CarrierId

$projection refers to an element of the current CDS projection:

$projection.CarrierId

So:

on $projection.CarrierId = _Carrier.AirlineID

means:

current entity CarrierId
=
associated carrier AirlineID

This keeps the relationship aligned with the current CDS model's element names.

---

14. Complete target-key reasoning

/DMO/I_CARRIER is identified by:

AirlineID

The association matches:

CarrierId → AirlineID

That is a complete target-key relationship.

For a to-one association, checking the target key is essential.

You will see a composite target key in the practice assignment.

---

15. Use the association with a path expression

Add:

_Carrier.Name as CarrierName,

This path expression contains:

_Carrier
  ↓
Name

Read it as:

Navigate through _Carrier and use the target's Name element.

The resulting scalar element in your entity is:

CarrierName

---

16. Activate and preview

Run:

Ctrl + F2
Ctrl + F3
F8

You should now see:

CarrierId
CarrierName

CarrierName is retrieved through the association path.

---

17. What happens when a path target field is used?

The association itself defines the relationship.

When you request a target field:

_Carrier.Name

the relationship must be instantiated to retrieve the target data.

Current CDS behavior uses a left outer join by default for a path target field in a CDS element list.

Conceptually:

association declaration
   =
relationship model

path target field
   =
use relationship to retrieve target value

---

18. Join vs association

Lesson 4:

Connection
  INNER JOIN Carrier

The join is fixed into the source expression.

Lesson 5:

Connection
  association → Carrier

The relationship can then be:

used through a path
exposed for navigation
reused by consumers

Associations are still translated into database operations when used. They are a higher-level relationship model, not a replacement for relational execution.

---

19. Add _OriginAirport

Add:

association [1..1] to /DMO/I_Airport as _OriginAirport
  on $projection.OriginAirportId = _OriginAirport.AirportID

The relationship is:

OriginAirportId → AirportID

---

20. Add Origin path fields

Add:

_OriginAirport.Name        as OriginAirportName,
_OriginAirport.City        as OriginCity,
_OriginAirport.CountryCode as OriginCountryCode,

Activate and preview.

Verify that the airport ID and descriptive values match.

---

21. Add _DestinationAirport

Add:

association [1..1] to /DMO/I_Airport as _DestinationAirport
  on $projection.DestinationAirportId = _DestinationAirport.AirportID

The same target CDS entity is now used in another role.

Role-based names make the distinction clear:

_OriginAirport
_DestinationAirport

---

22. Add Destination path fields

Add:

_DestinationAirport.Name        as DestinationAirportName,
_DestinationAirport.City        as DestinationCity,
_DestinationAirport.CountryCode as DestinationCountryCode,

Activate and preview.

---

23. Model before association exposure

Your entity can now look like:

@AccessControl.authorizationCheck: #NOT_REQUIRED
@EndUserText.label: 'Route with Associations'

define view entity ZI_Route_Assoc_###
  as select from /dmo/connection as Connection

    association [1..1] to /DMO/I_Carrier as _Carrier
      on $projection.CarrierId = _Carrier.AirlineID

    association [1..1] to /DMO/I_Airport as _OriginAirport
      on $projection.OriginAirportId = _OriginAirport.AirportID

    association [1..1] to /DMO/I_Airport as _DestinationAirport
      on $projection.DestinationAirportId = _DestinationAirport.AirportID

{
  key Connection.carrier_id       as CarrierId,
  key Connection.connection_id    as ConnectionId,

      _Carrier.Name               as CarrierName,

      Connection.airport_from_id  as OriginAirportId,
      _OriginAirport.Name         as OriginAirportName,
      _OriginAirport.City         as OriginCity,
      _OriginAirport.CountryCode  as OriginCountryCode,

      Connection.airport_to_id    as DestinationAirportId,
      _DestinationAirport.Name        as DestinationAirportName,
      _DestinationAirport.City        as DestinationCity,
      _DestinationAirport.CountryCode as DestinationCountryCode,

      Connection.departure_time   as DepartureTime,
      Connection.arrival_time     as ArrivalTime,
      Connection.distance         as Distance,
      Connection.distance_unit    as DistanceUnit
}

This uses associations internally, but the relationships themselves are not yet exposed.

---

24. Expose an association

Add the association name to the element list:

_Carrier

Do the same for:

_OriginAirport
_DestinationAirport

Near the end of the element list:

      _Carrier,
      _OriginAirport,
      _DestinationAirport

Now the relationships themselves are part of the public CDS model.

---

25. Final ZI_ROUTE_ASSOC_###

@AccessControl.authorizationCheck: #NOT_REQUIRED
@EndUserText.label: 'Route with Associations'

define view entity ZI_Route_Assoc_###
  as select from /dmo/connection as Connection

    association [1..1] to /DMO/I_Carrier as _Carrier
      on $projection.CarrierId = _Carrier.AirlineID

    association [1..1] to /DMO/I_Airport as _OriginAirport
      on $projection.OriginAirportId = _OriginAirport.AirportID

    association [1..1] to /DMO/I_Airport as _DestinationAirport
      on $projection.DestinationAirportId = _DestinationAirport.AirportID

{
  key Connection.carrier_id       as CarrierId,
  key Connection.connection_id    as ConnectionId,

      _Carrier.Name               as CarrierName,

      Connection.airport_from_id  as OriginAirportId,
      _OriginAirport.Name         as OriginAirportName,
      _OriginAirport.City         as OriginCity,
      _OriginAirport.CountryCode  as OriginCountryCode,

      Connection.airport_to_id    as DestinationAirportId,
      _DestinationAirport.Name        as DestinationAirportName,
      _DestinationAirport.City        as DestinationCity,
      _DestinationAirport.CountryCode as DestinationCountryCode,

      Connection.departure_time   as DepartureTime,
      Connection.arrival_time     as ArrivalTime,
      Connection.distance         as Distance,
      Connection.distance_unit    as DistanceUnit,

      _Carrier,
      _OriginAirport,
      _DestinationAirport
}

Check and activate:

Ctrl + F2
Ctrl + F3

---

26. Scalar path field vs exposed association

Compare:

_Carrier.Name as CarrierName

with:

_Carrier

The first creates a scalar result element:

CarrierName

The second exposes the relationship itself.

Think:

_Carrier.Name
=
give this entity a CarrierName field

_Carrier
=
make the Carrier relationship navigable

---

27. Follow an association in Data Preview

Open Data Preview:

F8

Now use the ADT association-navigation feature.

Step 27.1

Open:

Follow Association

from Data Preview.

Depending on ADT layout/version, this can be available from the table context menu or navigation/breadcrumb controls.

Step 27.2

Choose:

_Carrier

ADT navigates to the associated target data.

You should see target fields such as:

AirlineID
Name
CurrencyCode

Step 27.3

Use the breadcrumb/navigation control to return to:

ZI_ROUTE_ASSOC_###

Step 27.4

Follow:

_OriginAirport

Inspect:

AirportID
Name
City
CountryCode

Step 27.5

Repeat for:

_DestinationAirport

TIP

An exposed association is now visible as an actual navigation relationship in the development tooling.

---

28. Why exposed associations matter

Without exposing _Carrier, your entity can still use:

_Carrier.Name

internally.

Exposing:

_Carrier

adds the relationship itself to the entity's public model.

That matters later for:

other CDS models
projection layers
service navigation
RAP models

---

29. Path expressions can be longer

Conceptually, path expressions can navigate through more than one association:

_Association1._Association2.Field

For example:

Booking
  ↓ _Customer
Customer
  ↓ _Country
Country

The exact allowed use depends on cardinality, context, filters, and path-expression rules.

This lesson deliberately uses one association step at a time.

---

30. To-one vs to-many

Our guided associations are to-one:

Connection → Carrier
Connection → Origin Airport
Connection → Destination Airport

A different direction can be to-many:

Carrier → Connections

One carrier can have several connections.

That could be modeled as:

[0..*]

The practice includes this as an optional challenge.

---

31. Why cardinality matters

Do not treat cardinality as a comment.

It influences how CDS understands the relationship and can influence generated SQL/optimization behavior.

Before declaring to-one, ask:

Can zero targets exist?
Can multiple targets exist?
Does the ON condition match the target key?

---

32. Path target join attributes

The default CDS path target field uses left outer join semantics.

Current CDS also supports a path attribute such as:

_Carrier[inner].Name

to request inner-join behavior for that path.

Do not change the lesson model to this form.

The important point is to understand that:

association cardinality

and:

join behavior when a path is used

are related but different concepts.

---

33. SQL Console

Open SQL Console and query:

SELECT FROM zi_route_assoc_###
  FIELDS CarrierId,
         ConnectionId,
         CarrierName,
         OriginAirportId,
         OriginCity,
         DestinationAirportId,
         DestinationCity
  ORDER BY CarrierId,
           ConnectionId

The consumer sees ordinary scalar elements.

It does not repeat the associations or path expressions.

---

34. Filter a path-derived element

Use a country code that exists in your tenant.

Example:

SELECT FROM zi_route_assoc_###
  FIELDS CarrierId,
         ConnectionId,
         CarrierName,
         OriginCity,
         OriginCountryCode,
         DestinationCity,
         DestinationCountryCode
  WHERE OriginCountryCode = 'DE'
  ORDER BY CarrierId,
           ConnectionId

OriginCountryCode was derived through:

_OriginAirport.CountryCode

but the SQL consumer uses the exposed scalar name.

---

35. Create ZCL_ROUTE_ASSOC_READER_###

Create a normal ABAP class.

Do not implement:

IF_OO_ADT_CLASSRUN

Define:

CLASS zcl_route_assoc_reader_### DEFINITION
  PUBLIC
  FINAL
  CREATE PUBLIC.

  PUBLIC SECTION.

    TYPES tt_routes
      TYPE STANDARD TABLE OF zi_route_assoc_###
      WITH EMPTY KEY.

ENDCLASS.

---

36. Add Reader methods

Add:

METHODS get_all
  RETURNING VALUE(routes) TYPE tt_routes.

METHODS get_from_country
  IMPORTING
    country_code TYPE /dmo/airport-country
  RETURNING
    VALUE(routes) TYPE tt_routes.

---

37. Implement GET_ALL

METHOD get_all.

  SELECT FROM zi_route_assoc_###
    FIELDS *
    ORDER BY CarrierId,
             ConnectionId
    INTO TABLE @routes.

ENDMETHOD.

The Reader contains no relationship definition.

---

38. Implement GET_FROM_COUNTRY

METHOD get_from_country.

  SELECT FROM zi_route_assoc_###
    FIELDS *
    WHERE OriginCountryCode = @country_code
    ORDER BY CarrierId,
             ConnectionId
    INTO TABLE @routes.

ENDMETHOD.

A scalar path-derived CDS field can be filtered like any other exposed element.

---

39. Complete Reader

CLASS zcl_route_assoc_reader_### DEFINITION
  PUBLIC
  FINAL
  CREATE PUBLIC.

  PUBLIC SECTION.

    TYPES tt_routes
      TYPE STANDARD TABLE OF zi_route_assoc_###
      WITH EMPTY KEY.

    METHODS get_all
      RETURNING VALUE(routes) TYPE tt_routes.

    METHODS get_from_country
      IMPORTING
        country_code TYPE /dmo/airport-country
      RETURNING
        VALUE(routes) TYPE tt_routes.

ENDCLASS.


CLASS zcl_route_assoc_reader_### IMPLEMENTATION.

  METHOD get_all.

    SELECT FROM zi_route_assoc_###
      FIELDS *
      ORDER BY CarrierId,
               ConnectionId
      INTO TABLE @routes.

  ENDMETHOD.


  METHOD get_from_country.

    SELECT FROM zi_route_assoc_###
      FIELDS *
      WHERE OriginCountryCode = @country_code
      ORDER BY CarrierId,
               ConnectionId
      INTO TABLE @routes.

  ENDMETHOD.

ENDCLASS.

Activate it.

---

40. Create the Runner

Create:

ZCL_CDS_L05_###

Definition:

CLASS zcl_cds_l05_### DEFINITION
  PUBLIC
  FINAL
  CREATE PUBLIC.

  PUBLIC SECTION.
    INTERFACES if_oo_adt_classrun.

ENDCLASS.

---

41. Consume the Reader

CLASS zcl_cds_l05_### IMPLEMENTATION.

  METHOD if_oo_adt_classrun~main.

    DATA(reader) = NEW zcl_route_assoc_reader_###( ).

    DATA(routes) = reader->get_all( ).

    LOOP AT routes INTO DATA(route).

      out->write(
        |{ route-CarrierName }: | &&
        |{ route-OriginCity } -> { route-DestinationCity }|
      ).

    ENDLOOP.

  ENDMETHOD.

ENDCLASS.

Activate and run:

F9

---

42. Test the Reader filter

Choose a country code present in Data Preview.

Example:

DATA country_code TYPE /dmo/airport-country VALUE 'DE'.

DATA(routes_from_country) =
  reader->get_from_country(
    country_code = country_code
  ).

out->write( routes_from_country ).

Use a value that exists in your own tenant.

---

43. Associations are part of CDS, not RAP behavior

A normal CDS association models a data relationship.

It is not:

a RAP action
a validation
a determination
a service binding

RAP later uses CDS relationships heavily, but the association itself is a CDS modeling concept.

---

44. Associations vs compositions

Later RAP lessons introduce:

composition
association to parent

Those are specialized relationship concepts for business-object hierarchies.

This lesson focuses on normal associations:

association ... to ...

---

45. Common mistakes

Using target persistence names

Target:

/DMO/I_CARRIER

exposes:

AirlineID
Name
CurrencyCode

Do not write:

carrier_id
currency_code

against the CDS target.

---

Forgetting $projection

The current model alias:

CarrierId

is referenced as:

$projection.CarrierId

inside the association condition.

---

Using the wrong airport relationship

Wrong:

_OriginAirport.Name as DestinationAirportName

Correct:

_DestinationAirport.Name as DestinationAirportName

---

Declaring every relationship [1..1]

Choose cardinality based on the real relationship.

---

Declaring to-one without matching the target key

If several target rows can match, the relationship is not really to-one.

---

Forgetting that path target fields perform data access

This:

_Carrier.Name

causes target data to be retrieved.

The association is not merely documentation.

---

Forgetting to expose the association

This:

_Carrier.Name as CarrierName

does not automatically expose the _Carrier relationship.

Add:

_Carrier

if navigation should be public.

---

Assuming associations are automatically bidirectional

They are directional.

---

46. Knowledge check

1. What does an association define?

Show answer

A named directional relationship from a CDS source to a target, together with cardinality and an ON condition.

2. What does [1..1] mean?

Show answer

Exactly one target record is expected for one source record.

3. What does [0..*] mean?

Show answer

Zero, one, or many target records may exist for one source record.

4. What is $projection.CarrierId?

Show answer

A reference to the current CDS entity's projected element CarrierId.

5. What does _Carrier.Name mean?

Show answer

Navigate the _Carrier association and use the target's Name element.

6. What is the default join behavior of a CDS path target field in the element list?

Show answer

Left outer join semantics.

7. What does _Carrier by itself in the element list do?

Show answer

It exposes the association so supported consumers can navigate the relationship.

8. Are associations automatically bidirectional?

Show answer

No.

9. Why should a to-one association match the complete target key?

Show answer

Otherwise several target records may match one source record, contradicting the declared relationship.

10. Why did joins come before associations in this course?

Show answer

Joins make row matching, ON conditions, join types, and row multiplication explicit. Associations then add a reusable relationship abstraction over those concepts.

---

47. Lesson summary

You moved from:

fixed explicit joins

to:

named reusable relationships

You learned:

association
source and target
association aliases
cardinality
ON conditions
$projection
path expressions
path target fields
association exposure
Follow Association in ADT
default left outer path behavior
Reader consumption

The final model is:

/DMO/CONNECTION
      ↓
ZI_ROUTE_ASSOC_###
      ├── _Carrier [1..1] ───────────► /DMO/I_CARRIER
      ├── _OriginAirport [1..1] ─────► /DMO/I_AIRPORT
      └── _DestinationAirport [1..1] ► /DMO/I_AIRPORT

The central idea is:

A CDS association gives a relationship a name, cardinality, condition, and navigation path so it can be reused instead of existing only as a fixed join.

---

48. What comes next?

Before moving on, complete the Lesson 5 practice assignment to apply associations to booking, customer, carrier, and connection data. Use the Lesson 5 companion for deeper explanations, troubleshooting, and reference implementations.

The model now contains:

data
calculations
filters
joins
associations
navigation

Lesson 6 — CDS Semantics & Annotations adds semantic metadata:

amount + currency
quantity + unit
text elements
text associations
annotation propagation
business meaning

That prepares the CDS layer for later service consumption.