diff --git a/lib/indexes.js b/lib/indexes.js
new file mode 100644
index 0000000..d39ece0
--- /dev/null
+++ b/lib/indexes.js
@@ -0,0 +1,24 @@
+var BinarySearchTree = require('binary-search-tree').BinarySearchTree;
+
+
+/**
+ * Create a new index
+ * @param {String} options.fieldName On which field should the index apply
+ * @param {Datastore} options.datastore Datastore on which the index is created
+ * @param {Boolean} options.unique Optional, enforce a unique constraint (default: false)
+ * @param {Boolean} options.sparse Optional, allow a sparse index (we can have documents for which fieldName is undefined) (default: false)
+ */
+function Index (options) {
+  this.fieldName = options.fieldName;
+  this.datastore = options.datastore;
+  this.unique = options.unique || false;
+  this.sparse = options.unique || false;
+
+  if (this.sparse) { this.fieldUndefined = []; }   // Will store all elements for which the indexed field is not defined
+}
+
+
+
+
+// Interface
+module.exports = Index;
diff --git a/lib/model.js b/lib/model.js
index e8819c7..d46df86 100644
--- a/lib/model.js
+++ b/lib/model.js
@@ -226,6 +226,7 @@ function modify (obj, updateQuery) {
 
   // Check result is valid and return it
   checkObject(newDoc);
+  if (obj._id !== newDoc._id) { throw "You can't change a document's _id"; }
   return newDoc;
 };
 
diff --git a/test/db.test.js b/test/db.test.js
index a978b37..95bff89 100644
--- a/test/db.test.js
+++ b/test/db.test.js
@@ -817,7 +817,7 @@ describe('Database', function () {
 
     it('Cant change the _id of a document', function (done) {
       d.insert({ a: 2 }, function (err, newDoc) {
-        d.update({ a: 2 }, { _id: 'nope' }, {}, function (err) {
+        d.update({ a: 2 }, { a: 2, _id: 'nope' }, {}, function (err) {
           assert.isDefined(err);
 
           d.find({}, function (err, docs) {
@@ -826,7 +826,18 @@ describe('Database', function () {
             docs[0].a.should.equal(2);
             docs[0]._id.should.equal(newDoc._id);
 
-            done();
+            d.update({ a: 2 }, { $set: { _id: 'nope' } }, {}, function (err) {
+              assert.isDefined(err);
+
+              d.find({}, function (err, docs) {
+                docs.length.should.equal(1);
+                Object.keys(docs[0]).length.should.equal(2);
+                docs[0].a.should.equal(2);
+                docs[0]._id.should.equal(newDoc._id);
+
+                done();
+              });
+            });
           });
         });
       });