data-select.html

<link rel="import" href="data-element-behaviors.html">

<dom-module id="data-select">
    <script>
        Polymer({
            is: 'data-select',
            extends: 'select',
            properties: {
                placeholder: String
            },
            behaviors: [
                DataElementBehaviors.Basic,
                DataElementBehaviors.OptionList
            ],
            ready: function() {
                label = this.labelField;
                _id = this.valueField;
                if(this.placeholder) {
                    var item = {};
                    item[_id] = "";
                    item[label] = this.placeholder;
                    this.items.unshift(item);
                }
                this.selectedItem = this.selectedItem || this.items[0];
            },

            get label() { return this.labelField; },
            set label(val) { this.label = val; },
            get _id() { return this.valueField; },
            set _id(val) { this._id = val; },

            attached: function() {
                this.items.forEach(function(item) {
                    var $option = document.createElement('option');
                    $option.innerHTML = item[label];
                    $option.value = item[_id];
                    if(this.selectedItem[_id] == item[_id]) $option.selected = true;
                    this.appendChild($option);
                }.bind(this));
            },

            validate: function() {
                if(this.required && this.value == '') {
                    this.setValidity(false, 'Please select an option.');
                    return false;
                } else if(this.validity.valid) this._doAdditionalValidations();
                return this.valid;
            },            

            _onChange: function(evt) {
                var newVal = {};
                console.log(this.value);
                newVal[_id] = this.value;
                this.selectedItem = newVal;
                this.validate();
            }
        });
    </script>
</dom-module>