02. Creating the extension

Create the extension project

Create a new folder reCAP25extension in the location of your choice.

Note

An Extension Project is a normal CAP project and can be run locally just like any other CAP application. The only difference is, that such project do not have their own data model, but pull a base modelfrom the host application in order to extend it. The CDS toolchain has some built-in convenience to achieve this. For integrated testing, some application developers prefer to put the extension project as a subfolder of the application itself (e.g. using _ to create a top-level position) but this does not reflect the reality of extension developers, who do not have access to the base application itself

Using a terminal, cds init will create the necessary folder structure for our extension project. Modify the package.json to reflect this:

{
  "name": "reCAP25extension",
  "extends": "@capire/incidents",
  "version": "1.0.0",
  "dependencies": {
    "@sap/cds": "^9",
    "@sap/cds-oyster": "^0.1.0",
    "express": "^4"
  },
  "engines": {
    "node": ">=20"
  },
  "devDependencies": {
    "@cap-js/sqlite": "^2"
  },
  "cds": {
    "requires": {
      "code-extensibility": {
        "runtime": "debug",
        "maxTime": 1000,
        "maxMemory": 4
      },
      "auth": {
        "[development]": {
          "users": {
            "alice": {
              "tenant": "t1",
              "roles": [
                "support",
                "admin",
                "cds.ExtensionDeveloper",
                "cds.Subscriber"
              ]
            },
            "bob": {
              "roles": [
                "support"
              ]
            }
          }
        }
      }
    }
  },
  "scripts": {
    "start": "cds-serve"
  },
  "workspaces": [
    ".base"
  ]
}

Note the following important parts: A workspace, which points to .base and the extends clause providing a resolvable name for the target application. The extension project also needs the CDS-OYSTER plugin as dependency, and code-extensibility turned on, for local testing and debugging (see later section).

The extension project is jumpstarted by pulling the base model using

cds pull --from http://localhost:4005 -u alice:

and running

npm i

to move the base model from .base to a folder in node_modules according to the name given in the extends clause of package.json.

In the db folder of the extension project, create a file extension.cds with the following content:

using { sap.capire.incidents as my } from '@capire/incidents';

extend my.Customers with {
  status: String(8)  @title: 'Customer Status';
};  

You can push this extension using

cds push --to localhost:4005 -u alice:

and test it by playing around the AdminService endpoints.

Some additional convenience

In order to make testing simpler, you can use cds add http to create useful testing requests. This will create files in the test/http folder. To test changes on the data model, the AdminService tests are more convenient, since there is no draft support enabled there. If you are using VS Code, the REST Client extension allows you to run the requests directly from the editor.

In the Third exercise — Extending the model, we will start implementing the use case.