appwrite/docs/examples/1.6.x/server-nodejs/examples/databases/create-relationship-attribute.md

const sdk = require('node-appwrite');

const client = new sdk.Client()
    .setEndpoint('https://<REGION>.cloud.appwrite.io/v1') // Your API Endpoint
    .setProject('<YOUR_PROJECT_ID>') // Your project ID
    .setKey('<YOUR_API_KEY>'); // Your secret API key

const databases = new sdk.Databases(client);

const result = await databases.createRelationshipAttribute(
    '<DATABASE_ID>', // databaseId
    '<COLLECTION_ID>', // collectionId
    '<RELATED_COLLECTION_ID>', // relatedCollectionId
    sdk.RelationshipType.OneToOne, // type
    false, // twoWay (optional)
    '', // key (optional)
    '', // twoWayKey (optional)
    sdk.RelationMutate.Cascade // onDelete (optional)
);