Can now have an _id which is 0

lib/datastore.js

@@ -312,7 +312,9 @@ Datastore.prototype.prepareDocumentForInsertion = function (newDoc) {
     preparedDoc = [];
     newDoc.forEach(function (doc) { preparedDoc.push(self.prepareDocumentForInsertion(doc)); });
   } else {
-    newDoc._id = newDoc._id || this.createNewId();
+    if (newDoc._id === undefined) {
+      newDoc._id = this.createNewId();
+    }
     preparedDoc = model.deepCopy(newDoc);
     model.checkObject(preparedDoc);
   }

test/db.test.js

@@ -268,6 +268,14 @@ describe('Database', function () {
       });
     });
 
+    it('Can insert a doc with id 0', function (done) {
+      d.insert({ _id: 0, hello: 'world' }, function (err, doc) {
+        doc._id.should.equal(0);
+        doc.hello.should.equal('world');
+        done();
+      });
+    });
+    
     /**
      * Complicated behavior here. Basically we need to test that when a user function throws an exception, it is not caught
      * in NeDB and the callback called again, transforming a user error into a NeDB error.