Dont throw an error if updateQuery contains an _id that is the same as obj

lib/model.js

@@ -184,7 +184,7 @@ function modify (obj, updateQuery) {
     , newDoc, modifiers
     ;
 
-  if (keys.indexOf('_id') !== -1) { throw "You cannot change a document's _id"; }
+  if (keys.indexOf('_id') !== -1 && updateQuery._id !== obj._id) { throw "You cannot change a document's _id"; }
 
   if (dollarFirstChars.length !== 0 && dollarFirstChars.length !== firstChars.length) {
     throw "You cannot mix modifiers and normal fields";

package.json

@@ -1,6 +1,6 @@
 {
   "name": "nedb",
-  "version": "0.3.0",
+  "version": "0.3.1",
   "author": {
     "name": "tldr.io",
     "email": "hello@tldr.io"

test/model.test.js

@@ -212,7 +212,7 @@ describe('Model', function () {
       t._id.should.equal('keepit');
     });
 
-    it('Throw an error if trying to replace the _id field in a copy-type modification', function () {
+    it('Throw an error if trying to change the _id field in a copy-type modification', function () {
       var obj = { some: 'thing', _id: 'keepit' }
         , updateQuery = { replace: 'done', bloup: [ 1, 8], _id: 'donttryit' }
         ;
@@ -220,6 +220,9 @@ describe('Model', function () {
       (function () {
         model.modify(obj, updateQuery);
       }).should.throw();
+
+      updateQuery._id = 'keepit';
+      model.modify(obj, updateQuery);   // No error thrown
     });
 
     it('Throw an error if trying to use modify in a mixed copy+modify way', function () {