flutter and firestore : change price and size dynamicly

Question

hello iam using flutter and firebase in my project, i have a product and evert product have a price, every product have a list of sizes, i want the product to change its price when the size changes. here is my product document:

i know that i have to change the prices into list but how to link them together in firebase or flutter. this is my code:

 Column(
                  children: <Widget>[
                    Padding(
                      padding: const EdgeInsets.all(0),
                      child: Row(
                        children: <Widget>[
                          Padding(
                            padding: const EdgeInsets.symmetric(horizontal: 8),
                            child: CustomText(
                              text: "Select a Size",
                              color: Colors.white,
                            ),
                          ),
                          Padding(
                            padding: const EdgeInsets.symmetric(horizontal: 8),
                            child: DropdownButton<String>(
                                value: _size,
                                style: TextStyle(color: Colors.white),
                                items: widget.product.sizes
                                    .map<DropdownMenuItem<String>>(
                                        (value) => DropdownMenuItem(
                                            value: value,
                                            child: CustomText(
                                              text: value,
                                              color: Colors.red,
                                            )))
                                    .toList(),
                                onChanged: (value) {
                                  setState(() {
                                    _size = value;
                                  });
                                }),
                          )
                        ],
                      ),
                    ),
                    Expanded(
                      child: Padding(
                        padding: const EdgeInsets.all(8.0),
                        child: Text(
                            "${widget.product.description}",
                            style: TextStyle(color: Colors.white)),
                      ),
                    ),
                    Padding(
                      padding: const EdgeInsets.all(9),
                      child: Material(
                          borderRadius: BorderRadius.circular(15.0),
                          color: Colors.white,
                          elevation: 0.0,
                          child: MaterialButton(
                            onPressed: () async {
                              appProvider.changeIsLoading();
                              bool success = await userProvider.addToCart(
                                  product: widget.product,
                                  size: _size);
                              if (success) {
                               toast("Added to Cart!");
                                userProvider.reloadUserModel();
                                appProvider.changeIsLoading();
                                return;
                              } else {
                                toast("Not added to Cart!");
                                appProvider.changeIsLoading();
                                return;
                              }
                            },
                            minWidth: MediaQuery.of(context).size.width,
                            child: appProvider.isLoading
                                ? Loading()
                                : Text(
                                    "Add to cart",
                                    textAlign: TextAlign.center,
                                    style: TextStyle(
                                        color: Colors.black,
                                        fontWeight: FontWeight.bold,
                                        fontSize: 20.0),
                                  ),
                          )),
                    ),
                    SizedBox(
                      height: 20,
                    )
                  ],
                ),
              ),
            )
          ],

Answer 1

There is trigger for widget:

onChanged: (value) {
  setState(() {
    _size = value;
    reference.setData({quantity : value}, merge: true)
  });
}),

Just change the reference to document path.