The modifier works as expected and is tested

lib/model.js

@@ -143,7 +143,34 @@ modifierFunctions.$set = function (obj, field, value) {
   }
 };
 
-modifierFunctions.$inc = function () {};
+
+/**
+ * Increase (or decrease) a 'number' field
+ * Create and initialize it if needed
+ * @param {Object} obj The model to set a field for
+ * @param {String} field Can contain dots, in that case that means we will set a subfield recursively
+ * @param {Model} value
+ */
+modifierFunctions.$inc = function (obj, field, value) {
+  var fieldParts = field.split('.');
+
+  if (typeof value !== 'number') { throw value + " must be a number"; }
+
+  if (fieldParts.length === 1) {
+    if (typeof obj[field] !== 'number') {
+      if (!_.has(obj, field)) {
+        obj[field] = value;
+      } else {
+        throw "Don't use the $inc modifier on non-number fields";
+      }
+    } else {
+      obj[field] += value;
+    }
+  } else {
+    obj[fieldParts[0]] = obj[fieldParts[0]] || {};
+    modifierFunctions.$inc(obj[fieldParts[0]], fieldParts.slice(1).join('.'), value);
+  }
+};
 
 
 /**

test/model.test.js

@@ -250,7 +250,6 @@ describe('Model', function () {
     });
 
     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' } }
@@ -285,7 +284,42 @@ describe('Model', function () {
 
         _.isEqual(modified, { yup: { subfield: 'changed', yop: 'yes indeed' }, totally: { doesnt: { exist: 'now it does' } } }).should.equal(true);
       });
+    });
+
+    describe('$inc modifier', function () {
+      it('Throw an error if you try to use it with a non-number or on a non number field', function () {
+        (function () {
+        var obj = { some: 'thing', yup: 'yes', nay: 2 }
+          , updateQuery = { $inc: { nay: 'notanumber' } }
+          , modified = model.modify(obj, updateQuery);
+        }).should.throw();
+
+        (function () {
+        var obj = { some: 'thing', yup: 'yes', nay: 'nope' }
+          , updateQuery = { $inc: { nay: 1 } }
+          , modified = model.modify(obj, updateQuery);
+        }).should.throw();
+      });
 
+      it('Can increment number fields or create and initialize them if needed', function () {
+        var obj = { some: 'thing', nay: 40 }
+          , modified;
+
+        modified = model.modify(obj, { $inc: { nay: 2 } });
+        _.isEqual(modified, { some: 'thing', nay: 42 }).should.equal(true);
+
+        // Incidentally, this tests that obj was not modified
+        modified = model.modify(obj, { $inc: { inexistent: -6 } });
+        _.isEqual(modified, { some: 'thing', nay: 40, inexistent: -6 }).should.equal(true);
+      });
+
+      it('Works recursively', function () {
+        var obj = { some: 'thing', nay: { nope: 40 } }
+          , modified;
+
+        modified = model.modify(obj, { $inc: { "nay.nope": -2, "blip.blop": 123 } });
+        _.isEqual(modified, { some: 'thing', nay: { nope: 38 }, blip: { blop: 123 } }).should.equal(true);
+      });
     });
 
   });   // ==== End of 'Modifying documents' ==== //