default value in subarray

[schsj]

Hi, I have simple scheme with subarray. Tell me please, how can I set random id to each element of them? result: http://prntscr.com/adg79g

var randId={
type:String,
default:getRand
};
var testShema=mongoose.Schema({
    field:randId,
    object:{field1:randId,field2:randId},
    array:[randId]
})

I try to use formageField like this https://github.com/node4good/formage/wiki/Custom-Fields , but randId of each element are equal. case result: http://prntscr.com/adgfcd
UPD:
Resolved by schema setter, but question with defaul value within arrow still opened

var ReadOnlyWidget = formage.widgets.TextWidget.extend({
    render: function (res) {
        this.attrs.readonly='readonly';
        this._super(res);
    }
});

var ReadOnly = formage.fields.StringField.extend({
    init: function (options) {
        options = options || {};
        options.widget = ReadOnlyWidget;
        this._super(options);
    }
});

var randId={
        type:String, 
        formageField:ReadOnly, 
        set: function(v){return v||getRand();
};