flutter and firestore : change price and size dynamicly - firebase

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.