New modifier

lib/model.js

@@ -237,6 +237,17 @@ lastStepModifierFunctions.$push = function (obj, field, value) {
   obj[field].push(value);
 };
 
+// Add an element to an array field only if it is not already in it
+// No modification if the element is already in the array
+// Note that it doesn't check whether the original array contains duplicates
+lastStepModifierFunctions.$addToSet = function (obj, field, value) {
+  // Create the array if it doesn't exist
+  if (!obj.hasOwnProperty(field)) { obj[field] = []; }
+
+  if (!util.isArray(obj[field])) { throw "Can't $addToSet an element on non-array values"; }
+  if (obj[field].indexOf(value) === -1) { obj[field].push(value); }
+};
+
 // Increment a numeric field's value
 lastStepModifierFunctions.$inc = function (obj, field, value) {
   if (typeof value !== 'number') { throw value + " must be a number"; }

test/model.test.js

@@ -378,6 +378,39 @@ describe('Model', function () {
 
     });   // End of '$push modifier'
 
+    describe('$addToSet modifier', function () {
+
+      it('Can add an element to a set', function () {
+        var obj = { arr: ['hello'] }
+          , modified;
+
+        modified = model.modify(obj, { $addToSet: { arr: 'world' } });
+        assert.deepEqual(modified, { arr: ['hello', 'world'] });
+
+        obj = { arr: ['hello'] };
+        modified = model.modify(obj, { $addToSet: { arr: 'hello' } });
+        assert.deepEqual(modified, { arr: ['hello'] });
+      });
+
+      it('Can add an element to a non-existent set and will create the array', function () {
+        var obj = { arr: [] }
+          , modified;
+
+        modified = model.modify(obj, { $addToSet: { arr: 'world' } });
+        assert.deepEqual(modified, { arr: ['world'] });
+      });
+
+      it('Throw if we try to addToSet to a non-array', function () {
+        var obj = { arr: 'hello' }
+          , modified;
+
+        (function () {
+          modified = model.modify(obj, { $addToSet: { arr: 'world' } });
+        }).should.throw();
+      });
+
+    });   // End of '$addToSet modifier'
+
   });   // ==== End of 'Modifying documents' ==== //