How to get aggregated data in moor_flutter?

Question

Let's say I have 2 simple tables Users and Orders:

• Users has columns Id

• Orders has columns Id and UserId

How do I get all orders of a user easily using moor_flutter and return it as a stream of the following model?

class UserModel {
  final String id;
  final List<OrderModel> orderModels;

  UserModel(this.id, this.orders);
}

class OrderModel {
  final String id;

  OrderModel(this.id);
}

This is the official documentation but it is not covering this use case.

Equivalent call with EFCore and C# would be:

public async Task<IEnumerable<UserModel>> GetUserOrders(String userId)
{
    return await _db.Users
        .Include(user  => user.Orders)
        .Where(user => user.Id == userId)
        .Select(user => new UserModel
        {
            Id = user.Id,
            OrderModels = user.Orders.Select(order => new OrderModel
            {
                Id = null,
            }).ToList()
        })
    .ToListAsync();
}

Answer 1

I faced the same problem recently. I was confused to use Joins as per the given example in documentation.

What I have done is:

The created class (in database file only. see below example) has two objects which you want to join(combine). I wrote the query in a class of database.

You will get better understanding with example:

@UseMoor(
  tables: [OfflineProductMasters, OfflineProductWiseStocks],
)
class MyDatabase extends _$MyDatabase {
  // we tell the database where to store the data with this constructor
  MyDatabase() : super(_openConnection());

  // you should bump this number whenever you change or add a table definition. Migrations
  // are covered later in this readme.

  @override
  int get schemaVersion => 1;

  Future<List<ProductWithStock>> getProductWiseStock(String searchString, int mappingOfflineSalesTypeId) async {
    try {
      final rows = await (select(offlineProductMasters).join([innerJoin(offlineProductWiseStocks, offlineProductMasters.itemId.equalsExp(offlineProductWiseStocks.itemId))])
        ..where(offlineProductWiseStocks.salesTypeId.equals(mappingOfflineSalesTypeId) & offlineProductMasters.productName.like('$searchString%'))
        ..limit(50)
      ).get();
      return rows.map((row) {
        return ProductWithStock(
          row.readTable(offlineProductMasters),
          row.readTableOrNull(offlineProductWiseStocks),
        );
      }).toList();
    }catch(exception){
      print(exception);
      return Future.value([]);
    }
  }
}

class ProductWithStock {
  final OfflineProductMaster offlineProductMaster;

  final OfflineProductWiseStock? productWithStock;

  ProductWithStock(this.offlineProductMaster, this.productWithStock);
}

Now you got the structure that how you can use this type of query. Hope you will write your query in this way.

I don't know whether you have solved it or not. If solved then please post the answer so others can get help.

Thank you.

Answer 2

This feels like a hacky workaround but what I ended up doing is that I created 2 classes called HabitWithLogs and HabitModel. I put my query result into HabitWithLogs instances and then group them into HabitModel instances.

Data classes:

class HabitWithLog {
  final Habit habit;
  final HabitLog? habitLog;

  HabitWithLog({required this.habit, required this.habitLog}) : assert(habitLog == null || habitLog.habitId == habit.id);
}

class HabitModel {
  final Habit habit;
  final List<HabitLog> habitLogs;

  HabitModel({required this.habit, required this.habitLogs});
}

Dao method:

Future<List<HabitModel>> getAllHabits() async {
  // Get habits and order
  final query0 = (_db.select(_db.habits)..orderBy([(t) => OrderingTerm(expression: t.order, mode: OrderingMode.asc)]));

  // Join with habit logs
  final query1 = query0.join([
    leftOuterJoin(_db.habitLogs, _db.habitLogs.habitId.equalsExp(_db.habits.id)),
  ]);

  // Naive way that return the same habit multiple times
  final hwlList = query1.map((rows) => HabitWithLog(
        habit: rows.readTable(_db.habits),
        habitLog: rows.readTableOrNull(_db.habitLogs),
      ));

  // Group hwlList by habits
  final groups = (await hwlList.get()).groupListsBy((hwl) => hwl.habit);

  // Map grouping
  return groups.entries
      .map((group) => HabitModel(
            habit: group.key,
            habitLogs: (group.value[0].habitLog == null) ? List<HabitLog>.empty() : group.value.map((hwl) => hwl.habitLog!).toList(),
          ))
      .toList();
}

Mapping the stream feels terrible and should not be the only way to achieve this.