Async/Await/then in Dart/Flutter

In simple words:

await is meant to interrupt the process flow until the async method has finished.
then however does not interrupt the process flow (meaning the next instructions will be executed) but enables you to run code when the async method is finished.

In your example, you cannot achieve what you want when you use then because the code is not ‘waiting’ and the return statement is processed and thus returns an empty list.

When you add the await, you explicitly say: ‘don’t go further until my Future method is completed (namely the then part).

You could write your code as follows to achieve the same result using only await:

Future<List<Expense>> getExpensesByFundId(int fundId) async {
    Database db = await database;

    List<Expense> expenseList = List();

    List<Map<String,dynamic>> expList = await db.query(expTable,where: '$expTable.$expFundId = $fundId');
    expList.forEach((Map<String, dynamic> expMap) {
        expenseList.add(Expense.fromMap(expMap));
    });

    return expenseList;
}

You could also choose to use only the then part, but you need to ensure that you call getExpensesByFundId properly afterwards:

Future<List<Expense>> getExpensesByFundId(int fundId) async {
    Database db = await database;

    List<Expense> expenseList = List();
    
    return db.query(expTable,where: '$expTable.$expFundId = $fundId')
        .then((List<Map<String,dynamic>> expList){
      expList.forEach((Map<String, dynamic> expMap){
        expenseList.add(Expense.fromMap(expMap));
      });
    });
}

// call either with an await
List<Expense> list = await getExpensesByFundId(1);
// or with a then (knowing that this will not interrupt the process flow and process the next instruction
getExpensesByFundId(1).then((List<Expense> l) { /*...*/ });