Warn when a database class isn't used as a singleton

extras/integration_tests/tests/lib/suite/suite.dart

@@ -14,6 +14,8 @@ abstract class TestExecutor {
 }
 
 void runAllTests(TestExecutor executor) {
+  moorRuntimeOptions.dontWarnAboutMultipleDatabases = true;
+
   tearDown(() async {
     await executor.deleteData();
   });

moor/lib/src/runtime/api/db_base.dart

@@ -1,5 +1,14 @@
 part of 'runtime_api.dart';
 
+/// Keep track of how many databases have been opened for a given database
+/// type.
+/// We get a number of error reports of "moor not generating tables" that have
+/// their origin in users opening multiple instances of their database. This
+/// can cause a race conditions when the second [GeneratedDatabase] is opening a
+/// underlying [DatabaseConnection] that is already opened but doesn't have the
+/// tables created.
+Map<Type, int> _openedDbCount = {};
+
 /// A base class for all generated databases.
 abstract class GeneratedDatabase extends DatabaseConnectionUser
     with QueryEngine {
@@ -33,12 +42,38 @@ abstract class GeneratedDatabase extends DatabaseConnectionUser
       {StreamQueryStore streamStore})
       : super(types, executor, streamQueries: streamStore) {
     executor?.databaseInfo = this;
+    assert(_handleInstantiated());
   }
 
   /// Used by generated code to connect to a database that is already open.
   GeneratedDatabase.connect(DatabaseConnection connection)
       : super.fromConnection(connection) {
     connection?.executor?.databaseInfo = this;
+    assert(_handleInstantiated());
+  }
+
+  bool _handleInstantiated() {
+    if (!_openedDbCount.containsKey(runtimeType) ||
+        moorRuntimeOptions.dontWarnAboutMultipleDatabases) {
+      _openedDbCount[runtimeType] = 1;
+      return true;
+    }
+    final count = ++_openedDbCount[runtimeType];
+    if (count > 1) {
+      print(
+        'WARNING (moor): It looks like you\'ve created the database '
+        '$runtimeType multiple times. When these two databases use the same '
+        'QueryExecutor, race conditions will ocur and might corrupt the '
+        'database. \n'
+        'Try to follow the advice at https://moor.simonbinder.eu/faq/#using-the-database '
+        'or, if you know what you\'re doing, set moorRuntimeOptions.dontWarnAboutMultipleDatabases = true\n'
+        'Here is the stacktrace from when the database was opened a second '
+        'time:\n${StackTrace.current}\n'
+        'This warning will only appear on debug builds.',
+      );
+    }
+
+    return true;
   }
 
   /// Creates a [Migrator] with the provided query executor. Migrators generate
@@ -89,5 +124,6 @@ abstract class GeneratedDatabase extends DatabaseConnectionUser
   /// Closes this database and releases associated resources.
   Future<void> close() async {
     await executor.close();
+    _openedDbCount[runtimeType]--;
   }
 }

moor/lib/src/runtime/api/runtime_api.dart

@@ -9,3 +9,16 @@ part 'connection.dart';
 part 'db_base.dart';
 part 'dao_base.dart';
 part 'query_engine.dart';
+
+/// Defines additional runtime behavior for moor. Changing the fields of this
+/// class is rarely necessary.
+class MoorRuntimeOptions {
+  /// Don't warn when a database class isn't used as singleton.
+  bool dontWarnAboutMultipleDatabases = false;
+}
+
+/// Stores the [MoorRuntimeOptions] describing global moor behavior across
+/// databases.
+///
+/// Note that is is adapting this behavior is rarely needed.
+MoorRuntimeOptions moorRuntimeOptions = MoorRuntimeOptions();

moor/test/data/tables/custom_tables.dart

@@ -7,7 +7,9 @@ part 'custom_tables.g.dart';
   queries: {'writeConfig': 'REPLACE INTO config VALUES (:key, :value)'},
 )
 class CustomTablesDb extends _$CustomTablesDb {
-  CustomTablesDb(QueryExecutor e) : super(e);
+  CustomTablesDb(QueryExecutor e) : super(e) {
+    moorRuntimeOptions.dontWarnAboutMultipleDatabases = true;
+  }
 
   @override
   int get schemaVersion => 1;

moor/test/data/tables/todos.dart

@@ -99,8 +99,12 @@ class CustomConverter extends TypeConverter<MyCustomObject, String> {
   },
 )
 class TodoDb extends _$TodoDb {
-  TodoDb(QueryExecutor e) : super(e);
-  TodoDb.connect(DatabaseConnection connection) : super.connect(connection);
+  TodoDb(QueryExecutor e) : super(e) {
+    moorRuntimeOptions.dontWarnAboutMultipleDatabases = true;
+  }
+  TodoDb.connect(DatabaseConnection connection) : super.connect(connection) {
+    moorRuntimeOptions.dontWarnAboutMultipleDatabases = true;
+  }
 
   @override
   MigrationStrategy get migration => MigrationStrategy();

moor/test/database_test.dart

@@ -32,6 +32,8 @@ class _FakeDb extends GeneratedDatabase {
 }
 
 void main() {
+  moorRuntimeOptions.dontWarnAboutMultipleDatabases = true;
+
   test('status of OpeningDetails', () {
     expect(const OpeningDetails(null, 1).wasCreated, true);
     expect(const OpeningDetails(2, 4).wasCreated, false);