By default, all attributes except 'createdAt' can be updated

lib/dialects/mysql/query-generator.js

@@ -198,7 +198,7 @@ module.exports = (function() {
         ')');
       }.bind(this));
 
-      if (options && options.updateOnDuplicate instanceof Array && options.updateOnDuplicate.length) {
+      if (options && options.updateOnDuplicate) {
         onDuplicateKeyUpdate += ' ON DUPLICATE KEY UPDATE ' + options.updateOnDuplicate.map(function(attr) {
           var key = this.quoteIdentifier(attr);
           return key + '=VALUES(' + key + ')';

lib/model.js

@@ -1232,7 +1232,7 @@ module.exports = (function() {
    * @param  {Boolean}      [options.hooks=true]             Run before / after bulk create hooks?
    * @param  {Boolean}      [options.individualHooks=false]  Run before / after create hooks for each individual Instance? BulkCreate hooks will still be run if options.hooks is true.
    * @param  {Boolean}      [options.ignoreDuplicates=false] Ignore duplicate values for primary keys? (not supported by postgres)
-   * @param  {Boolean}      [options.updateOnDuplicate]      Fields to update if row key already exists (on duplicate key update)? (only supported by mysql & mariadb)
+   * @param  {Array}        [options.updateOnDuplicate]      Fields to update if row key already exists (on duplicate key update)? (only supported by mysql & mariadb). By default, all fields are updated.
    *
    * @return {Promise<Array<Instance>>}
    */
@@ -1266,6 +1266,15 @@ module.exports = (function() {
       return Promise.reject(new Error(dialect + ' does not support the \'updateOnDuplicate\' option.'));
     }
 
+    if (options.updateOnDuplicate) {
+      // By default, all attributes except 'createdAt' can be updated
+      var updatableFields = Utils._.pull(Object.keys(this.tableAttributes), 'createdAt');
+      if (Utils._.isArray(options.updateOnDuplicate) && !Utils._.isEmpty(options.updateOnDuplicate)) {
+        updatableFields = Utils._.intersection(updatableFields, options.updateOnDuplicate);
+      }
+      options.updateOnDuplicate = updatableFields;
+    }
+
     var self = this
       , createdAtAttr = this._timestampAttributes.createdAt
       , updatedAtAttr = this._timestampAttributes.updatedAt