Check if Field Already exists in Flutter Firestore

Question

I have a collection called company. All the companies are going to be stored like in my screenshot.

When I add another company, I want

Answer 1

final QuerySnapshot result =
    await Firestore.instance.collection('users').where('nickname', isEqualTo: 
    nickname).getDocuments();

final List < DocumentSnapshot > documents = result.documents;

if (documents.length > 0) { 

  //exists

} else {  

  //not exists

}