- DOI implementation wit unit testing

app/Models/DatasetIdentifier.php

@@ -0,0 +1,19 @@
+<?php
+namespace App\Models;
+
+use App\Models\Dataset;
+use Illuminate\Database\Eloquent\Model;
+
+class DatasetIdentifier extends Model
+{
+    protected $table = 'dataset_identifierss';
+    protected $guarded = array();
+
+    /**
+     * The dataset that belong to the DocumentIdentifier.
+     */
+    public function dataset()
+    {
+        return $this->belongsTo(Dataset::class, 'document_id', 'id');
+    }
+}