How do I implement a Decodable trait for the inner field

I have the Encodable trait for Dao struct right, but how do I implement the Decodable trait to read the BTreeMap back and assign it into the inner field of the struct?

pub struct Dao {
    pub values: BTreeMap<String, Value>,

/// custom Encoder for Dao,
/// decodes directly the content of `values`, instead of `values` as field of this `Dao` struct
impl Encodable for Dao {
    fn encode<S: Encoder>(&self, s: &mut S) -> Result<(), S::Error> {

impl Decodable for Dao{
	fn decode<D: Decoder>(d: &mut D) -> Result<Self, D::Error> {
			let map: BTreeMap<String, Value> = Decodable::decode(a).unwrap();// this is invalid here
                        let dao = Dao{values: map};

The Json String for the BTreeMap

            "active": true,
            "barcode": null,
            "client_id": null,
            "created": "2015-11-19T17:10:42.246382+00:00",
            "created_by": null,
            "currency_id": null,
            "description": "Second hand Iphone4s",
            "help": null,
            "info": null,
            "is_service": false,
            "name": "iphone4s",
            "organization_id": null,
            "owner_id": "3e51d5f9-5bff-4664-9946-47bf37973636",
            "parent_product_id": null,
            "price": 7000,
            "priority": null,
            "product_id": "f7521093-734d-488a-9f60-fc9f11f7e750",
            "seq_no": null,
            "tags": null,
            "unit": null,
            "updated": "2015-11-19T17:10:42.246382+00:00",
            "updated_by": null,
            "upfront_fee": 0,
            "use_parent_price": false

This should do it

fn decode<D: Decoder>(d: &mut D) -> Result<Self, D::Error> {
    Ok(Dao {
        values: try!(Decodable::decode(d))

Thanks, that worked. I never thought it would be that simple.