Add support for nested relationship serialisation

Author: huntie

src/Exceptions/InvalidRelationPathException.php

@@ -0,0 +1,22 @@
+<?php
+
+namespace Huntie\JsonApi\Exceptions;
+
+use Exception;
+
+class InvalidRelationPathException extends Exeption
+{
+    /**
+     * Create a new InvalidRelationPathException instance.
+     *
+     * @param string         $path     The relation path attempted
+     * @param int|null       $code     User defined exception code
+     * @param Exception|null $previous Previous exception if nested
+     */
+    public function __construct($path, $code = 0, Exception $previous = null)
+    {
+        $message = sprintf('The relationship path "%s" could not be resolved', $path);
+
+        parent::__construct($message, $code, $previous);
+    }
+}

src/Serializers/RelationshipSerializer.php

@@ -2,6 +2,8 @@
 
 namespace Huntie\JsonApi\Serializers;
 
+use Exception;
+use Huntie\JsonApi\Exceptions\InvalidRelationPathException;
 use Illuminate\Database\Eloquent\Model;
 use Illuminate\Support\Collection;
 
@@ -24,15 +26,26 @@ class RelationshipSerializer extends JsonApiSerializer
     /**
      * Create a new JSON API relationship serializer.
      *
-     * @param Model      $record   The primary record
-     * @param string     $relation The named relation to serialize
-     * @param array|null $fields   Subset of fields to return by record type
+     * @param Model      $record The primary record
+     * @param string     $path   The path to the relation to serialize
+     * @param array|null $fields Subset of fields to return by record type
+     *
+     * @throws InvalidRelationPathException
      */
-    public function __construct($record, $relation, array $fields = [])
+    public function __construct($record, $path, array $fields = [])
     {
         parent::__construct();
 
-        $this->relation = $record->{$relation};
+        try {
+            $this->relation = $record;
+
+            foreach (explode('.', $path) as $relation) {
+                $this->relation = $this->relation->{$relation};
+            }
+        } catch (Exception $e) {
+            throw new InvalidRelationPathException($path);
+        }
+
         $this->fields = array_unique($fields);
     }