fix(schema): enforce that _id is never null

Re: #5236 mongodb/node-mongodb-native#517

lib/model.js

@@ -126,7 +126,7 @@ Model.prototype.$__handleSave = function(options, callback) {
 
     var obj = this.toObject(toObjectOptions);
 
-    if (!utils.object.hasOwnProperty(obj || {}, '_id')) {
+    if ((obj || {})._id == null) {
       // documents must have an _id else mongoose won't know
       // what to update later if more changes are made. the user
       // wouldn't know what _id was generated by mongodb either

lib/schema/objectid.js

@@ -196,6 +196,9 @@ function defaultId() {
 }
 
 function resetId(v) {
+  if (v == null) {
+    return new oid();
+  }
   this.$__._id = null;
   return v;
 }

test/document.test.js

@@ -4162,6 +4162,16 @@ describe('document', function() {
       });
     });
 
+    it('null _id (gh-5236)', function(done) {
+      var childSchema = new mongoose.Schema({});
+
+      var M = db.model('gh5236', childSchema);
+
+      var m = new M({ _id: null });
+      assert.ok(m._id);
+      done();
+    });
+
     it('modify multiple subdoc paths (gh-4405)', function(done) {
       var ChildObjectSchema = new Schema({
         childProperty1: String,