const relationships = await client.relationships.retrieve([{externalId: 'abc'}, {externalId: 'def'}]);

res = client.relationships.retrieve_multiple(external_ids=["abc", "def"])

List<Item> byExternalIds = List.of(Item.newBuilder() 
          .setExternalId("10") 
          .build()); 
List<Relationship> list = 
          client.relationships().retrieve(byExternalIds);//by list of items 
List<Relationship> list = 
          client.relationships().retrieve("10", "20");//by varargs of String 

List<Item> byInternalIds = List.of(Item.newBuilder() 
          .setId(10) 
          .build()); 
List<Relationship> list = 
          client.relationships().retrieve(byInternalIds, true);//by list of items 
List<Relationship> list1 = 
          client.relationships().retrieve(10,20);//by varargs of Long

curl --request POST \
  --url https://{cluster}.cognitedata.com/api/v1/projects/{project}/relationships/byids \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "items": [
    {
      "externalId": "<string>"
    }
  ],
  "ignoreUnknownIds": false,
  "fetchResources": false
}
'

<?php

$curl = curl_init();

curl_setopt_array($curl, [
  CURLOPT_URL => "https://{cluster}.cognitedata.com/api/v1/projects/{project}/relationships/byids",
  CURLOPT_RETURNTRANSFER => true,
  CURLOPT_ENCODING => "",
  CURLOPT_MAXREDIRS => 10,
  CURLOPT_TIMEOUT => 30,
  CURLOPT_HTTP_VERSION => CURL_HTTP_VERSION_1_1,
  CURLOPT_CUSTOMREQUEST => "POST",
  CURLOPT_POSTFIELDS => json_encode([
    'items' => [
        [
                'externalId' => '<string>'
        ]
    ],
    'ignoreUnknownIds' => false,
    'fetchResources' => false
  ]),
  CURLOPT_HTTPHEADER => [
    "Authorization: Bearer <token>",
    "Content-Type: application/json"
  ],
]);

$response = curl_exec($curl);
$err = curl_error($curl);

curl_close($curl);

if ($err) {
  echo "cURL Error #:" . $err;
} else {
  echo $response;
}

package main

import (
	"fmt"
	"strings"
	"net/http"
	"io"
)

func main() {

	url := "https://{cluster}.cognitedata.com/api/v1/projects/{project}/relationships/byids"

	payload := strings.NewReader("{\n  \"items\": [\n    {\n      \"externalId\": \"<string>\"\n    }\n  ],\n  \"ignoreUnknownIds\": false,\n  \"fetchResources\": false\n}")

	req, _ := http.NewRequest("POST", url, payload)

	req.Header.Add("Authorization", "Bearer <token>")
	req.Header.Add("Content-Type", "application/json")

	res, _ := http.DefaultClient.Do(req)

	defer res.Body.Close()
	body, _ := io.ReadAll(res.Body)

	fmt.Println(string(body))

}

require 'uri'
require 'net/http'

url = URI("https://{cluster}.cognitedata.com/api/v1/projects/{project}/relationships/byids")

http = Net::HTTP.new(url.host, url.port)
http.use_ssl = true

request = Net::HTTP::Post.new(url)
request["Authorization"] = 'Bearer <token>'
request["Content-Type"] = 'application/json'
request.body = "{\n  \"items\": [\n    {\n      \"externalId\": \"<string>\"\n    }\n  ],\n  \"ignoreUnknownIds\": false,\n  \"fetchResources\": false\n}"

response = http.request(request)
puts response.read_body

{
  "items": [
    {
      "externalId": "<string>",
      "sourceExternalId": "<string>",
      "targetExternalId": "<string>",
      "createdTime": 1730204346000,
      "lastUpdatedTime": 1730204346000,
      "startTime": 1730204346000,
      "endTime": 1730204346000,
      "confidence": 0.5,
      "dataSetId": 4503599627370496,
      "labels": [
        {
          "externalId": "my.known.id"
        }
      ],
      "source": {
        "createdTime": 1730204346000,
        "lastUpdatedTime": 1730204346000,
        "rootId": 4503599627370496,
        "name": "<string>",
        "id": 4503599627370496,
        "aggregates": {
          "childCount": 1,
          "depth": 1,
          "path": [
            {
              "id": 4503599627370496
            }
          ]
        },
        "parentId": 4503599627370496,
        "parentExternalId": "my.known.id",
        "externalId": "my.known.id",
        "description": "<string>",
        "dataSetId": 4503599627370496,
        "metadata": {},
        "source": "<string>",
        "labels": [
          {
            "externalId": "my.known.id"
          }
        ],
        "geoLocation": {
          "type": "Feature",
          "geometry": {
            "type": "Point",
            "coordinates": [
              123
            ]
          },
          "properties": {}
        }
      },
      "target": {
        "createdTime": 1730204346000,
        "lastUpdatedTime": 1730204346000,
        "rootId": 4503599627370496,
        "name": "<string>",
        "id": 4503599627370496,
        "aggregates": {
          "childCount": 1,
          "depth": 1,
          "path": [
            {
              "id": 4503599627370496
            }
          ]
        },
        "parentId": 4503599627370496,
        "parentExternalId": "my.known.id",
        "externalId": "my.known.id",
        "description": "<string>",
        "dataSetId": 4503599627370496,
        "metadata": {},
        "source": "<string>",
        "labels": [
          {
            "externalId": "my.known.id"
          }
        ],
        "geoLocation": {
          "type": "Feature",
          "geometry": {
            "type": "Point",
            "coordinates": [
              123
            ]
          },
          "properties": {}
        }
      }
    }
  ]
}

{
  "error": {
    "code": 409,
    "message": "Could not authenticate.",
    "missing": [
      {}
    ],
    "duplicated": [
      {}
    ]
  }
}

{
  "error": {
    "code": 409,
    "message": "Could not authenticate.",
    "missing": [
      {}
    ],
    "duplicated": [
      {}
    ]
  }
}