diff --git a/lib/model.js b/lib/model.js
index 70f7d6e..cb20c09 100644
--- a/lib/model.js
+++ b/lib/model.js
@@ -129,13 +129,18 @@ function deepCopy (obj) {
  * Set field to value in a model
  * Create it if it doesn't exist
  * @param {Object} obj The model to set a field for
- * @param {Object} mod mod is of the form { k: v }. k is the field, v its value
- *                     k can contain dots, in that case that means we will set a subfield
+ * @param {String} field Can contain dots, in that case that means we will set a subfield recursively
+ * @param {Model} value
  */
-modifierFunctions.$set = function (obj, mod) {
-  var field, value;
-
+modifierFunctions.$set = function (obj, field, value) {
+  var fieldParts = field.split('.');
 
+  if (fieldParts.length === 1) {
+    obj[field] = value;
+  } else {
+    obj[fieldParts[0]] = obj[fieldParts[0]] || {};
+    modifierFunctions.$set(obj[fieldParts[0]], fieldParts.slice(1).join('.'), value);
+  }
 };
 
 modifierFunctions.$inc = function () {};
@@ -176,8 +181,9 @@ function modify (obj, updateQuery) {
         throw "Modifier " + m + "'s argument must be an object";
       }
 
+      newDoc = deepCopy(obj);
       keys.forEach(function (k) {
-        modifierFunctions[m](obj, k, updateQuery[m][k]);
+        modifierFunctions[m](newDoc, k, updateQuery[m][k]);
       });
     });
   }
diff --git a/test/model.test.js b/test/model.test.js
index e2fc1dd..57d6c02 100644
--- a/test/model.test.js
+++ b/test/model.test.js
@@ -249,6 +249,45 @@ describe('Model', function () {
       }).should.throw();
     });
 
+    describe('$set modifier', function () {
+
+      it('Can change already set fields without modfifying the underlying object', function () {
+        var obj = { some: 'thing', yup: 'yes', nay: 'noes' }
+          , updateQuery = { $set: { some: 'changed', nay: 'yes indeed' } }
+          , modified = model.modify(obj, updateQuery);
+
+        Object.keys(modified).length.should.equal(3);
+        modified.some.should.equal('changed');
+        modified.yup.should.equal('yes');
+        modified.nay.should.equal('yes indeed');
+
+        Object.keys(obj).length.should.equal(3);
+        obj.some.should.equal('thing');
+        obj.yup.should.equal('yes');
+        obj.nay.should.equal('noes');
+      });
+
+      it('Creates fields to set if they dont exist yet', function () {
+        var obj = { yup: 'yes' }
+          , updateQuery = { $set: { some: 'changed', nay: 'yes indeed' } }
+          , modified = model.modify(obj, updateQuery);
+
+        Object.keys(modified).length.should.equal(3);
+        modified.some.should.equal('changed');
+        modified.yup.should.equal('yes');
+        modified.nay.should.equal('yes indeed');
+      });
+
+      it('Can set sub-fields and create them if necessary', function () {
+        var obj = { yup: { subfield: 'bloup' } }
+          , updateQuery = { $set: { "yup.subfield": 'changed', "yup.yop": 'yes indeed', "totally.doesnt.exist": 'now it does' } }
+          , modified = model.modify(obj, updateQuery);
+
+        _.isEqual(modified, { yup: { subfield: 'changed', yop: 'yes indeed' }, totally: { doesnt: { exist: 'now it does' } } }).should.equal(true);
+      });
+
+    });
+
   });   // ==== End of 'Modifying documents' ==== //
 
 });