<dom-module id="x-row">
        <template>
            <p>key=<span>{{model.key}}</span></p>
        </template>
    </dom-module>
    <script>
        Polymer({
            is : 'x-row',

            properties: {
                model: {
                    type: Object
                }
            }
        });
    </script>

    <dom-module id="x-list">
        <template>
            <template is="dom-repeat" items="[[model]]" as="groupData">
                <p>-----</p>
                <template is="dom-repeat" items="[[groupData]]" as="model">
                    <x-row model="[[model]]"></x-row>
                </template>
            </template>
        </template>
    </dom-module>
    <script>
        Polymer({
            is : 'x-list',

            properties: {
                model: {
                    type: Array,
                    value: [
                        [
                            { key:'aaaa' },
                            { key:'bbbb' },
                            { key:'cccc' }
                        ],
                        [
                            { key:'dddd' },
                        ]
                    ]
                }
            }
        });
    </script>

    <x-list id="test"></x-list>
    <input type="button" value="test" onclick="test();">
    <script>
        function test() {
            var test = document.getElementById('test');
            test.set('model.0.1.key', 'dddd');
        }
    </script>

      _modelForPath: function(path) {
        var dot = path.indexOf('.');    // <--- path is undefined
        return (dot < 0) ? path : path.slice(0, dot);
      },

                <template is="dom-repeat" items="[[groupData]]">
                    <x-row model="[[item]]"></x-row>
                </template>