diff --git a/lib/validator.js b/lib/validator.js
index d0bf2a4..4a02afd 100644
--- a/lib/validator.js
+++ b/lib/validator.js
@@ -42,6 +42,26 @@ ajv.addKeyword('relation', {
     }
 });
 
+ajv.addKeyword('$toJSON', {
+    modifying: true,
+    statements: true,
+    valid: true,
+    metaSchema: {
+        type: 'object'
+    },
+    validate: function(schema, data, parentSchema, dataPath, parentData, prop) {
+        if (   typeof data === 'object'
+            && data !== null
+            && typeof data.toJSON === 'function'
+            && parentData
+        ) {
+            parentData[prop] = data.toJSON();
+        } else if (typeof data === 'string') {
+            parentData[prop] = JSON.parse(data);
+        }
+    }
+});
+
 ajv.addKeyword('$toDate', {
     type: ['string', 'integer'],
     modifying: true,