Implement single() version for get() and watch()

moor/lib/src/runtime/database.dart

@@ -142,8 +142,7 @@ mixin QueryEngine on DatabaseConnectionUser {
   /// value.
   Future<List<QueryRow>> customSelect(String query,
       {List<Variable> variables = const []}) async {
-    return CustomSelectStatement(query, variables, <TableInfo>{}, this)
-        .execute();
+    return CustomSelectStatement(query, variables, <TableInfo>{}, this).get();
   }
 
   /// Creates a stream from a custom select statement.To use the variables, mark
@@ -155,7 +154,7 @@ mixin QueryEngine on DatabaseConnectionUser {
       {List<Variable> variables = const [], Set<TableInfo> readsFrom}) {
     final tables = readsFrom ?? <TableInfo>{};
     final statement = CustomSelectStatement(query, variables, tables, this);
-    return createStream(statement.constructFetcher());
+    return statement.watch();
   }
 
   /// Executes [action] in a transaction, which means that all its queries and

moor/lib/src/runtime/statements/query.dart

@@ -10,6 +10,7 @@ import 'package:moor/src/runtime/expressions/custom.dart';
 import 'package:moor/src/runtime/expressions/expression.dart';
 import 'package:moor/src/types/sql_types.dart';
 import 'package:moor/src/runtime/structure/table_info.dart';
+import 'package:moor/src/utils/single_transformer.dart';
 
 /// Statement that operates with data that already exists (select, delete,
 /// update).
@@ -68,6 +69,47 @@ abstract class Query<T extends Table, DataClass> {
   }
 }
 
+/// Abstract class for queries which can return one-time values or a stream
+/// of values.
+abstract class Selectable<T> {
+  /// Executes this statement and returns the result.
+  Future<List<T>> get();
+
+  /// Creates an auto-updating stream of the result that emits new items
+  /// whenever any table used in this statement changes.
+  Stream<List<T>> watch();
+
+  /// Executes this statement, like [get], but only returns one value. If the
+  /// result has no or too many values, this method will throw.
+  ///
+  /// Be aware that this operation won't put a limit clause on this statement,
+  /// if that's needed you would have to do that yourself.
+  Future<T> getSingle() async {
+    final list = await get();
+    final iterator = list.iterator;
+
+    if (!iterator.moveNext()) {
+      throw StateError('Expected exactly one result, but actually there were '
+          'none!');
+    }
+    final element = iterator.current;
+    if (iterator.moveNext()) {
+      throw StateError('Expected exactly one result, but found more than one!');
+    }
+
+    return element;
+  }
+
+  /// Creates an auto-updating stream of this statement, similar to [watch].
+  /// However, it is assumed that the query will only emit one result, so
+  /// instead of returning a [Stream<List<T>>], this returns a [Stream<T>]. If
+  /// the query emits more than one row at some point, an error will be emitted
+  /// to the stream instead.
+  Stream<T> watchSingle() {
+    return watch().transform(singleElements());
+  }
+}
+
 mixin SingleTableQueryMixin<T extends Table, DataClass> on Query<T, DataClass> {
   void where(Expression<bool, BoolType> filter(T tbl)) {
     final predicate = filter(table.asDslTable);

moor/lib/src/runtime/statements/select.dart

@@ -14,7 +14,8 @@ import 'package:moor/src/runtime/structure/table_info.dart';
 typedef OrderingTerm OrderClauseGenerator<T>(T tbl);
 
 class JoinedSelectStatement<FirstT extends Table, FirstD>
-    extends Query<FirstT, FirstD> with LimitContainerMixin {
+    extends Query<FirstT, FirstD>
+    with LimitContainerMixin, Selectable<TypedResult> {
   JoinedSelectStatement(
       QueryEngine database, TableInfo<FirstT, FirstD> table, this._joins)
       : super(database, table);
@@ -97,8 +98,7 @@ class JoinedSelectStatement<FirstT extends Table, FirstD>
     orderByExpr = OrderBy(terms);
   }
 
-  /// Creates an auto-updating stream of the result that emits new items
-  /// whenever any table of this statement changes.
+  @override
   Stream<List<TypedResult>> watch() {
     final ctx = constructQuery();
     final fetcher = QueryStreamFetcher<List<TypedResult>>(
@@ -110,7 +110,7 @@ class JoinedSelectStatement<FirstT extends Table, FirstD>
     return database.createStream(fetcher);
   }
 
-  /// Executes this statement and returns the result.
+  @override
   Future<List<TypedResult>> get() async {
     final ctx = constructQuery();
     return _getWithQuery(ctx);
@@ -143,7 +143,7 @@ class JoinedSelectStatement<FirstT extends Table, FirstD>
 
 /// A select statement that doesn't use joins
 class SimpleSelectStatement<T extends Table, D> extends Query<T, D>
-    with SingleTableQueryMixin<T, D>, LimitContainerMixin<T, D> {
+    with SingleTableQueryMixin<T, D>, LimitContainerMixin<T, D>, Selectable<D> {
   SimpleSelectStatement(QueryEngine database, TableInfo<T, D> table)
       : super(database, table);
 
@@ -155,7 +155,7 @@ class SimpleSelectStatement<T extends Table, D> extends Query<T, D>
     ctx.buffer.write('SELECT * FROM ${table.tableWithAlias}');
   }
 
-  /// Loads and returns all results from this select query.
+  @override
   Future<List<D>> get() async {
     final ctx = constructQuery();
     return _getWithQuery(ctx);
@@ -212,8 +212,7 @@ class SimpleSelectStatement<T extends Table, D> extends Query<T, D>
     orderByExpr = OrderBy(clauses.map((t) => t(table.asDslTable)).toList());
   }
 
-  /// Creates an auto-updating stream that emits new items whenever this table
-  /// changes.
+  @override
   Stream<List<D>> watch() {
     final query = constructQuery();
     final fetcher = QueryStreamFetcher<List<D>>(
@@ -228,7 +227,7 @@ class SimpleSelectStatement<T extends Table, D> extends Query<T, D>
 
 /// A select statement that is constructed with a raw sql prepared statement
 /// instead of the high-level moor api.
-class CustomSelectStatement {
+class CustomSelectStatement with Selectable<QueryRow> {
   /// Tables this select statement reads from. When turning this select query
   /// into an auto-updating stream, that stream will emit new items whenever
   /// any of these tables changes.
@@ -242,12 +241,21 @@ class CustomSelectStatement {
   final List<Variable> variables;
   final QueryEngine _db;
 
-  /// Constructs a new
+  /// Constructs a new custom select statement for the query, the variables,
+  /// the affected tables and the database.
   CustomSelectStatement(this.query, this.variables, this.tables, this._db);
 
   /// Constructs a fetcher for this query. The fetcher is responsible for
   /// updating a stream at the right moment.
+  @Deprecated(
+      'There is no need to use this method. Please use watch() directly')
   QueryStreamFetcher<List<QueryRow>> constructFetcher() {
+    return _constructFetcher();
+  }
+
+  /// Constructs a fetcher for this query. The fetcher is responsible for
+  /// updating a stream at the right moment.
+  QueryStreamFetcher<List<QueryRow>> _constructFetcher() {
     final args = _mapArgs();
 
     return QueryStreamFetcher<List<QueryRow>>(
@@ -257,11 +265,22 @@ class CustomSelectStatement {
     );
   }
 
-  /// Executes this query and returns the result.
-  Future<List<QueryRow>> execute() async {
+  @override
+  Future<List<QueryRow>> get() async {
     return _executeWithMappedArgs(_mapArgs());
   }
 
+  @override
+  Stream<List<QueryRow>> watch() {
+    return _db.createStream(_constructFetcher());
+  }
+
+  /// Executes this query and returns the result.
+  @Deprecated('Use get() instead')
+  Future<List<QueryRow>> execute() async {
+    return get();
+  }
+
   List<dynamic> _mapArgs() {
     final ctx = GenerationContext.fromDb(_db);
     return variables.map((v) => v.mapToSimpleValue(ctx)).toList();

moor/lib/src/utils/single_transformer.dart

@@ -0,0 +1,14 @@
+import 'dart:async';
+
+/// Transforms a stream of lists into a stream of single elements, assuming
+/// that each list is a singleton.
+StreamTransformer<List<T>, T> singleElements<T>() {
+  return StreamTransformer.fromHandlers(handleData: (data, sink) {
+    try {
+      sink.add(data.single);
+    } catch (e) {
+      sink.addError(
+          StateError('Expected exactly one element, but got ${data.length}'));
+    }
+  });
+}

moor/test/select_test.dart

@@ -6,6 +6,20 @@ import 'package:test_api/test_api.dart';
 import 'data/tables/todos.dart';
 import 'data/utils/mocks.dart';
 
+final _dataOfTodoEntry = {
+  'id': 10,
+  'title': 'A todo title',
+  'content': 'Content',
+  'category': 3
+};
+
+final _todoEntry = TodoEntry(
+  id: 10,
+  title: 'A todo title',
+  content: 'Content',
+  category: 3,
+);
+
 void main() {
   TodoDb db;
   MockExecutor executor;
@@ -78,20 +92,10 @@ void main() {
 
   group('SELECT results are parsed', () {
     test('when all fields are non-null', () {
-      final data = [
-        {'id': 10, 'title': 'A todo title', 'content': 'Content', 'category': 3}
-      ];
-      final resolved = TodoEntry(
-        id: 10,
-        title: 'A todo title',
-        content: 'Content',
-        category: 3,
-      );
-
       when(executor.runSelect('SELECT * FROM todos;', any))
-          .thenAnswer((_) => Future.value(data));
+          .thenAnswer((_) => Future.value([_dataOfTodoEntry]));
 
-      expect(db.select(db.todosTable).get(), completion([resolved]));
+      expect(db.select(db.todosTable).get(), completion([_todoEntry]));
     });
 
     test('when some fields are null', () {
@@ -116,4 +120,35 @@ void main() {
       expect(db.select(db.todosTable).get(), completion([resolved]));
     });
   });
+
+  group('queries for a single row', () {
+    test('get once', () {
+      when(executor.runSelect('SELECT * FROM todos;', any))
+          .thenAnswer((_) => Future.value([_dataOfTodoEntry]));
+
+      expect(db.select(db.todosTable).getSingle(), completion(_todoEntry));
+    });
+
+    test('get multiple times', () {
+      final resultRows = <List<Map<String, dynamic>>>[
+        [_dataOfTodoEntry],
+        [],
+        [_dataOfTodoEntry, _dataOfTodoEntry],
+      ];
+      var _currentRow = 0;
+
+      when(executor.runSelect('SELECT * FROM todos;', any)).thenAnswer((_) {
+        return Future.value(resultRows[_currentRow++]);
+      });
+
+      expectLater(
+          db.select(db.todosTable).watchSingle(),
+          emitsInOrder(
+              [_todoEntry, emitsError(anything), emitsError(anything)]));
+
+      db
+        ..markTablesUpdated({db.todosTable})
+        ..markTablesUpdated({db.todosTable});
+    });
+  });
 }

moor/test/utils/single_transformer_test.dart

@@ -0,0 +1,25 @@
+import 'dart:async';
+
+import 'package:moor/src/utils/single_transformer.dart';
+import 'package:test_api/test_api.dart';
+
+void main() {
+  test('transforms simple values', () {
+    final controller = StreamController<List<int>>();
+    final stream = controller.stream.transform(singleElements());
+
+    expectLater(stream, emitsInOrder([1, 2, 3, 4]));
+
+    controller..add([1])..add([2])..add([3])..add([4]);
+  });
+
+  test('emits errors for invalid lists', () {
+    final controller = StreamController<List<int>>();
+    final stream = controller.stream.transform(singleElements());
+
+    expectLater(stream,
+        emitsInOrder([1, emitsError(anything), 2, emitsError(anything)]));
+
+    controller..add([1])..add([2, 3])..add([2])..add([]);
+  });
+}