all models into model folder

app/Models/Collection.php

@@ -0,0 +1,15 @@
+<?php
+
+namespace App\Models;
+
+use Illuminate\Database\Eloquent\Model;
+use App\Models\Dataset;
+
+class Collection extends Model
+{
+
+    public function documents()
+    {
+        return $this->belongsToMany(Dataset::class, 'link_documents_collections', 'collection_id', 'document_id');
+    }
+}