Updating items in Firestore using Flutter

I am making a friend req type Functionality
So if I tap on add friend button the name of Current Logged in person should be added to the request list of person named I searched.

So from my app when I click on add friend button, let say current logged in user is ‘c@c.com’ and I search for ‘d@d.com’ then ‘c@c.com’ user name should be added in the requests array of the person ‘d@d.com’.

This is my search method to search a user

void onSearch() async {
    await _firestore
        .collection("users")
        .where("email", isEqualTo: _search.text)
        .get()
        .then((value) {
      setState(() {
        userMap = value.docs[0].data();
      });
    });
  }

What should I write in addUser method ?

void addUser() async {
    await _firestore
            .collection("users")
            .where("email", isEqualTo: _search.text)
            .get()
           ;
  }

>Solution :

To learn how to add items to and remove items from an array, see the Firebase documentation on updating an array. To learn how to update all documents that match a query, see How to get the document ref to a where clause firestore?