How to create a subcollection inside a collection's each document

Question

I am making an simple currency reminder app where I am supposed to note down the money I receive from other user or I gave other user money as a reminder so as to not forget it. And while creating a subcollection for each Collection's document, I think there is a problem.

The code for adding the received data into firebase is as follows:

onPressed: () async{
              Map<String, dynamic> mapdata = {
                'amount':_amountreceivedcontroller.text,
                'details': _detailsreceivedcontroller.text,
              };
              var collectiondata = await FirebaseFirestore.instance.collection('LoanFriends');
              var querySnapshots = await collectiondata.get();
              for(var snapshot in querySnapshots.docs){
                var documentID = snapshot.id;
                await FirebaseFirestore.instance.collection('LoanFriends').doc(documentID).collection('ReceivedLoanData').add(mapdata);
              }
              _amountreceivedcontroller.text = '';
              _detailsreceivedcontroller.text = '';
              Navigator.pop(context);
            },

Answer 1

onPressed: () async{
      Map<String, dynamic> mapdata = {
        'amount':_amountreceivedcontroller.text,
        'details': _detailsreceivedcontroller.text,
      };
      //Friend you lend money
      var frined = _friendController.text;
      var collectiondata = await FirebaseFirestore.instance.collection('LoanFriends');
      var querySnapshots = await collectiondata.get();
      for(var snapshot in querySnapshots.docs){
        var documentID = snapshot.id;
        //for the specific document associated with your friend
        if(snapshot.name==friend){
          //add lended money details and break loop
          await collectiondata.doc(documentID).collection('ReceivedLoanData').add(mapdata);
          break;
        }
      }
      _amountreceivedcontroller.text = '';
      _detailsreceivedcontroller.text = '';
      _friendController.text = '';
      Navigator.pop(context);
}

You can also query your firestore collections

onPressed: () async{
      Map<String, dynamic> mapdata = {
        'amount':_amountreceivedcontroller.text,
        'details': _detailsreceivedcontroller.text,
      };
      //Friend you lend money
      var frined = _friendController.text;
      var collectiondata = await FirebaseFirestore.instance.collection('LoanFriends');
      //directly query your database with friend name and get specific document
      var query = collectiondata.where("name", '==', friend);
      var snapshot = await query.get();

      //add lended money details
      await snapshot.collection('ReceivedLoanData').add(mapdata);
      
      _amountreceivedcontroller.text = '';
      _detailsreceivedcontroller.text = '';
      _friendController.text = '';
      Navigator.pop(context);
}