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

You can simply use a where query to only receive documents that have that name and then check whether you get documents. Here is an async example method that would perform what you want to know.

Example method

Future doesNameAlreadyExist(String name) async {
  final QuerySnapshot result = await Firestore.instance
    .collection('company')
    .where('name', isEqualTo: name)
    .limit(1)
    .getDocuments();
  final List documents = result.documents;
  return documents.length == 1;
}

As you can see, I am only receiving documents, where the name field matches the given name. I also add limit(1) to make sure that I do not unnecessarily retrieve more than 1 document (which would never happen in theory) and then I just check if the length of all documents in the company collection is equal to 1 or not. If it is equal to 1, there already is a company that has that name and otherwise not.

You could also remove the limit(1) and make the check documents.length > 1 and that would work too, but might retrieve unnecessary documents.

Example implementation

@override
Widget build(BuildContext context) {
  return FutureBuilder(
    future: doesNameAlreadyExist('nova'),
    builder: (context, AsyncSnapshot result) {
      if (!result.hasData)
        return Container(); // future still needs to be finished (loading)
      if (result.data) // result.data is the returned bool from doesNameAlreadyExists
        return Text('A company called "Nova" already exists.');
      else
        return Text('No company called "Nova" exists yet.');
    },
  );
}

Here, I am not displaying an error message, which would be easily possible with the example method as well. However, the build method of some widget is used. This would e.g. work in a dialog, but I decided to do it to keep it simple and understandable. The FutureBuilder takes in doesNameAlreadyExist, in this case with the name "Nova" from your question and will, in the end, return a Text widget stating whether the name already exists.

Be careful

The where query is case-sensitive. This means that the check would not work if you typed e.g. "noVa" instead of "nova". As this might be important to you, you can make use of this nice method, where you would create an extra field that is insensitive, e.g. all letters are small and then you would simple query like this:

.where('name_insensitive', isEqualTo: name.toLowerCase())