diff --git a/lib/model.js b/lib/model.js
index 46893fb..9f12a0e 100644
--- a/lib/model.js
+++ b/lib/model.js
@@ -6,6 +6,8 @@
 
 var dateToJSON = function () { return { $$date: this.getTime() }; }
   , originalDateToJSON = Date.prototype.toJSON
+  , util = require('util')
+  ;
 
 
 /**
@@ -23,6 +25,26 @@ function checkKey (k, v) {
 }
 
 
+/**
+ * Check a DB object and throw an error if it's not valid
+ * Works by applying the above checkKey function to all fields recursively
+ */
+function checkObject (obj) {
+  if (util.isArray(obj)) {
+    obj.forEach(function (o) {
+      checkObject(o);
+    });
+  }
+
+  if (typeof obj === 'object') {
+    Object.keys(obj).forEach(function (k) {
+      checkKey(k, obj[k]);
+      checkObject(obj[k]);
+    });
+  }
+}
+
+
 /**
  * Serialize an object to be persisted to a one-line string
  * Accepted primitive types: Number, String, Boolean, Date, null
@@ -88,7 +110,6 @@ function deepCopy (obj) {
   if (typeof obj === 'object') {
     res = {};
     Object.keys(obj).forEach(function (k) {
-      checkKey(k, obj[k]);
       res[k] = deepCopy(obj[k]);
     });
     return res;
@@ -105,6 +126,9 @@ function deepCopy (obj) {
 function modify (obj, updateQuery) {
   updateQuery = deepCopy(updateQuery);
   updateQuery._id = obj._id;
+
+  checkObject(updateQuery);
+
   return updateQuery;
 };
 
@@ -113,4 +137,5 @@ function modify (obj, updateQuery) {
 module.exports.serialize = serialize;
 module.exports.deserialize = deserialize;
 module.exports.deepCopy = deepCopy;
+module.exports.checkObject = checkObject;
 module.exports.modify = modify;
diff --git a/test/model.test.js b/test/model.test.js
index 914c934..42490f8 100644
--- a/test/model.test.js
+++ b/test/model.test.js
@@ -127,6 +127,30 @@ describe('Model', function () {
   });   // ==== End of 'Serialization, deserialization' ==== //
 
 
+  describe('Object checking', function () {
+
+    it('Field names beginning with a $ sign are forbidden', function () {
+      assert.isDefined(model.checkObject);
+
+      (function () {
+        model.checkObject({ $bad: true });
+      }).should.throw();
+
+      (function () {
+        model.checkObject({ some: 42, nested: { again: "no", $worse: true } });
+      }).should.throw();
+
+      // This shouldn't throw since "$actuallyok" is not a field name
+      model.checkObject({ some: 42, nested: [ 5, "no", "$actuallyok", true ] });
+
+      (function () {
+        model.checkObject({ some: 42, nested: [ 5, "no", "$actuallyok", true, { $hidden: "useless" } ] });
+      }).should.throw();
+    });
+
+  });   // ==== End of 'Object checking' ==== //
+
+
   describe('Deep copying', function () {
 
     it('Should be able to deep copy any serializable model', function () {
@@ -157,16 +181,6 @@ describe('Model', function () {
       res.subobj.b.should.equal('c');
     });
 
-    it('Will throw an error if obj contains a field beginning by the $ sign', function () {
-      (function () {
-        model.deepCopy({ $something: true });
-      }).should.throw();
-
-      (function () {
-        model.deepCopy({ something: true, another: { $badfield: 'rrr' } });
-      }).should.throw();
-    });
-
   });   // ==== End of 'Deep copying' ==== //
 
 });