diff --git a/lib/collection.js b/lib/collection.js
index 73d0b03..f320c5d 100644
--- a/lib/collection.js
+++ b/lib/collection.js
@@ -99,7 +99,9 @@ export default class Collection {
       }
     } else {
       if (options && options.upsert) {
-        data._id = UUID().replace(/-/g, '');
+        if ( !("_id" in data) ) {
+          data._id = UUID().replace(/-/g, '');
+        }
         collection.push(data);
         ret.updated = 0;
         ret.inserted = 1;
diff --git a/test/diskdb_test.js b/test/diskdb_test.js
index 5dd28aa..e698d4c 100644
--- a/test/diskdb_test.js
+++ b/test/diskdb_test.js
@@ -41,6 +41,11 @@ var dbPath = 'test/testdb',
         title: 'diskDB rocks',
         published: 'yesterday'
     },
+    article4 = {
+        _id : 'pre_generated_id',
+        title: 'diskDB rocks',
+        published: 'yesterday'
+    },
     //nested objects
     articleComments = {
         title: 'diskDB rocks',
@@ -375,7 +380,7 @@ exports.update = {
             'upsert': false
         };
 
-        test.expect(4);
+        test.expect(6);
         //save one record
         diskdb.articles.save(article);
         // before update
@@ -394,6 +399,11 @@ exports.update = {
         // should insert
         test.equal(diskdb.articles.update(query, article2, options).inserted, 1, 'Should return the inserted objects count');
 
+        // should insert with pre-generated _id
+        var previous_id = article4._id;
+        test.equal(diskdb.articles.update({_id: article4._id}, article4, options).inserted, 1, 'Should return the inserted objects count');
+        test.equal(article4._id, previous_id, 'Should have the pre-generated _id');
+
         //change options
         query = {
             published: 'yesterday'
@@ -405,7 +415,7 @@ exports.update = {
         };
 
         // should update 2 record
-        test.equal(diskdb.articles.update(query, article, options).updated, 2, 'Should return the updated objects count');
+        test.equal(diskdb.articles.update(query, article, options).updated, 3, 'Should return the updated objects count');
         test.done();
     },
 };