diff --git a/src/Jenssegers/Mongodb/Relations/BelongsToMany.php b/src/Jenssegers/Mongodb/Relations/BelongsToMany.php
index 2cca88638..37ae75e8d 100644
--- a/src/Jenssegers/Mongodb/Relations/BelongsToMany.php
+++ b/src/Jenssegers/Mongodb/Relations/BelongsToMany.php
@@ -179,6 +179,10 @@ public function attach($id, array $attributes = [], $touch = true)
             // Attach the new parent id to the related model.
             $model->push($this->foreignKey, $this->parent->getKey(), true);
         } else {
+            if ($id instanceof Collection) {
+                $id = $id->modelKeys();
+            }
+
             $query = $this->newRelatedQuery();
 
             $query->whereIn($this->related->getKeyName(), (array) $id);
diff --git a/tests/RelationsTest.php b/tests/RelationsTest.php
index 8572d4727..e0a734acc 100644
--- a/tests/RelationsTest.php
+++ b/tests/RelationsTest.php
@@ -291,6 +291,18 @@ public function testBelongsToManyAttachArray()
         $this->assertCount(2, $user->clients);
     }
 
+    public function testBelongsToManyAttachEloquentCollection()
+    {
+        $user = User::create(['name' => 'John Doe']);
+        $client1 = Client::create(['name' => 'Test 1']);
+        $client2 = Client::create(['name' => 'Test 2']);
+        $collection = new \Illuminate\Database\Eloquent\Collection([$client1, $client2]);
+
+        $user = User::where('name', '=', 'John Doe')->first();
+        $user->clients()->attach($collection);
+        $this->assertCount(2, $user->clients);
+    }
+
     public function testBelongsToManySyncAlreadyPresent()
     {
         $user = User::create(['name' => 'John Doe']);