Shopping carts in moor

shopping_cart.dart

import 'package:moor/moor.dart';
import 'package:moor/moor_vm.dart';
import 'package:rxdart/rxdart.dart';

part 'shopping_cart.g.dart';

class ShoppingCarts extends Table {
  IntColumn get id => integer().autoIncrement()();
}

class BuyableItems extends Table {
  IntColumn get id => integer().autoIncrement()();
// price, description, idk
}

@DataClassName('ShoppingCartEntry')
class ShoppingCartEntries extends Table {
  IntColumn get shoppingCart => integer()();
  IntColumn get item => integer()();
}

class CartWithItems {
  final ShoppingCart cart;
  final List<BuyableItem> items;

  CartWithItems(this.cart, this.items);
}

@UseMoor(tables: [ShoppingCarts, BuyableItems, ShoppingCartEntries])
class Database extends _$Database {
  @override
  final int schemaVersion = 1;

  Database(QueryExecutor e) : super(e);

  @override
  MigrationStrategy get migration {
    return MigrationStrategy(
      beforeOpen: (_, details) async {
        if (details.wasCreated) {
          // write some fake items
          for (var i = 0; i < 10; i++) {
            await into(buyableItems).insert(const BuyableItemsCompanion());
          }
        }
      },
    );
  }

  /// Create a new empty shopping cart
  Future<ShoppingCart> createShoppingCart() async {
    final id = await into(shoppingCarts).insert(const ShoppingCartsCompanion());
    return ShoppingCart(id: id);
  }

  Future<List<BuyableItem>> loadAllItems() async {
    return await select(buyableItems).get();
  }

  Stream<CartWithItems> watchCart(int id) {
    final cartQuery = select(shoppingCarts)
      ..where((cart) => cart.id.equals(id));

    final contentQuery = select(shoppingCartEntries).join(
      [
        innerJoin(
          buyableItems,
          buyableItems.id.equalsExp(shoppingCartEntries.item),
        ),
      ],
    )..where(shoppingCartEntries.shoppingCart.equals(id));

    final cartStream = cartQuery.watchSingle();

    final contentStream = contentQuery.watch().map((rows) {
      return rows.map((row) => row.readTable(buyableItems)).toList();
    });

    return Observable.combineLatest2(cartStream, contentStream,
        (ShoppingCart cart, List<BuyableItem> items) {
      return CartWithItems(cart, items);
    });
  }

  Stream<List<CartWithItems>> watchAllCarts() {
    // start by watching all carts
    final cartStream = Observable(select(shoppingCarts).watch());

    return cartStream.switchMap((carts) {
      // this method is called whenever the list of carts changes. For each
      // cart, we want to load all the items in it.
      // (we create a map from id to cart here just for performance reasons)
      final idToCart = {for (var cart in carts) cart.id: cart};
      final ids = idToCart.keys;

      // select all entries that are included in a cart that we found
      final entryQuery = select(shoppingCartEntries).join(
        [
          innerJoin(
            buyableItems,
            buyableItems.id.equalsExp(shoppingCartEntries.item),
          )
        ],
      )..where(isIn(shoppingCartEntries.shoppingCart, ids));

      return entryQuery.watch().map((rows) {
        // Store the list of entries for each cart, again using maps for faster
        // lookups.
        final idToItems = <int, List<BuyableItem>>{};

        // for each entry (row) that is included in a cart, put it in the map
        // of items.
        for (var row in rows) {
          final item = row.readTable(buyableItems);
          final id = row.readTable(shoppingCartEntries).shoppingCart;

          idToItems.putIfAbsent(id, () => []).add(item);
        }

        // finally, all that's left is to merge the map of carts with the map of
        // entries
        return [
          for (var id in ids)
            CartWithItems(idToCart[id], idToItems[id] ?? []),
        ];
      });
    });
  }

  Future<void> writeShoppingCart(CartWithItems entry) {
    return transaction((_) async {
      final cart = entry.cart;

      // first, we write the shopping cart
      await into(shoppingCarts).insert(cart, orReplace: true);

      // replace entries of the cart
      await (delete(shoppingCartEntries)
            ..where((entry) => entry.shoppingCart.equals(cart.id)))
          .go();

      await into(shoppingCartEntries).insertAll([
        for (var item in entry.items)
          ShoppingCartEntry(shoppingCart: cart.id, item: item.id),
      ]);
    });
  }
}

void main() async {
  final db = Database(VMDatabase.memory());

  final cart = await db.createShoppingCart();
  final availableItems = await db.loadAllItems();

  final sub = db.watchCart(cart.id).listen((updated) {
    print('Cart ${updated.cart} now consists of ${updated.items}');
  });

  await db.writeShoppingCart(CartWithItems(cart, availableItems.sublist(0, 3)));
  await db.writeShoppingCart(CartWithItems(cart, availableItems.sublist(3, 6)));
  await db.writeShoppingCart(CartWithItems(cart, availableItems.sublist(6)));

  await Future.delayed(const Duration(seconds: 2));
  await sub.cancel();
  await db.close();
}

I spend all my free time online shopping. I often look for bargains. And after reading this article I learned how to do it more efficiently. Now all my purchases are more thoughtful.