Writing queries

Learn how to write database queries in pure Dart with moor

Note: This assumes that you already completed the setup.

For each table you’ve specified in the @UseMoor annotation on your database class, a corresponding getter for a table will be generated. That getter can be used to run statements:

// inside the database class, the `todos` getter has been created by moor.
@UseMoor(tables: [Todos, Categories])
class MyDatabase extends _$MyDatabase {  

  // the schemaVersion getter and the constructor from the previous page
  // have been omitted.
  
  // loads all todo entries
  Future<List<Todo>> get allTodoEntries => select(todos).get();

  // watches all todo entries in a given category. The stream will automatically
  // emit new items whenever the underlying data changes.
  Stream<List<Todo>> watchEntriesInCategory(Category c) {
    return (select(todos)..where((t) => t.category.equals(c.id))).watch();
  }
}

Select statements

You can create select statements by starting them with select(tableName), where the table name is a field generated for you by moor. Each table used in a database will have a matching field to run queries against. Any query can be run once with get() or be turned into an auto-updating stream using watch().

Where

You can apply filters to a query by calling where(). The where method takes a function that should map the given table to an Expression of boolean. A common way to create such expression is by using equals on expressions. Integer columns can also be compared with isBiggerThan and isSmallerThan. You can compose expressions using a & b, a | b and a.not(). For more details on expressions, see this guide.

Limit

You can limit the amount of results returned by calling limit on queries. The method accepts the amount of rows to return and an optional offset.

Future<List<Todo>> limitTodos(int limit, {int offset}) {
  return (select(todos)..limit(limit, offset: offset)).get();
}

Ordering

You can use the orderBy method on the select statement. It expects a list of functions that extract the individual ordering terms from the table. You can use any expression as an ordering term - for more details, see this guide.

Future<List<Todo>> sortEntriesAlphabetically() {
  return (select(todos)..orderBy([(t) => OrderingTerm(expression: t.title)])).get();
}

You can also reverse the order by setting the mode property of the OrderingTerm to OrderingMode.desc.

Single values

If you know a query is never going to return more than one row, wrapping the result in a List can be tedious. Moor lets you work around that with getSingle and watchSingle:

Stream<Todo> entryById(int id) {
  return (select(todos)..where((t) => t.id.equals(id))).watchSingle();
}

If an entry with the provided id exists, it will be sent to the stream. Otherwise, null will be added to stream. If a query used with watchSingle ever returns more than one entry (which is impossible in this case), an error will be added instead.

Mapping

Before calling watch or get (or the single variants), you can use map to transform the result.

Stream<List<String>> contentWithLongTitles() {
  final query = select(todos)
    ..where((t) => t.title.length.isBiggerOrEqualValue(16));

  return query
    .map((row) => row.content)
    .watch();
}

If you need more complex queries with joins or custom columns, see this site.

Updates and deletes

You can use the generated classes to update individual fields of any row:

Future moveImportantTasksIntoCategory(Category target) {
  // for updates, we use the "companion" version of a generated class. This wraps the
  // fields in a "Value" type which can be set to be absent using "Value.absent()". This
  // allows us to separate between "SET category = NULL" (`category: Value(null)`) and not
  // updating the category at all: `category: Value.absent()`.
  return (update(todos)
      ..where((t) => t.title.like('%Important%'))
    ).write(TodosCompanion(
      category: Value(target.id),
    ),
  );
}

Future update(Todo entry) {
  // using replace will update all fields from the entry that are not marked as a primary key.
  // it will also make sure that only the entry with the same primary key will be updated.
  // Here, this means that the row that has the same id as entry will be updated to reflect
  // the entry's title, content and category. As it set's its where clause automatically, it
  // can not be used together with where.
  return update(todos).replace(entry);
}

Future feelingLazy() {
  // delete the oldest nine tasks
  return (delete(todos)..where((t) => t.id.isSmallerThanValue(10))).go();
}

⚠️ Caution: If you don’t explicitly add a where clause on updates or deletes, the statement will affect all rows in the table!

Inserts

You can very easily insert any valid object into tables. As some values can be absent (like default values that we don’t have to set explicitly), we again use the companion version.

// returns the generated id
Future<int> addTodo(TodosCompanion entry) {
  return into(todos).insert(entry);
}

All row classes generated will have a constructor that can be used to create objects:

addTodo(
  TodosCompanion(
    title: Value('Important task'),
    content: Value('Refactor persistence code'),
  ),
);

If a column is nullable or has a default value (this includes auto-increments), the field can be omitted. All other fields must be set and non-null. The insert method will throw otherwise.

Multiple insert statements can be run efficiently by using a batch. To do that, you can use the insertAll method inside a batch:

Future<void> insertMultipleEntries() async{
  await batch((batch) {
    // functions in a batch don't have to be awaited - just
    // await the whole batch afterwards.
    batch.insertAll(todos, [
      TodosCompanion.insert(
        title: 'First entry',
        content: 'My content',
      ),
      TodosCompanion.insert(
        title: 'Another entry',
        content: 'More content',
        // columns that aren't required for inserts are still wrapped in a Value:
        category: Value(3),
      ),
      // ...
    ]);
  });
}

Upserts

Upserts are a feature from newer sqlite3 versions that allows an insert to behave like an update if a conflicting row already exists.

This allows us to create or override an existing row when its primary key is part of its data:

class Users extends Table {
  TextColumn get email => text()();
  TextColumn get name => text()();

  @override
  Set<Column> get primaryKey => {email};
}

Future<void> createOrUpdateUser(User user) {
  return into(users).insertOnConflictUpdate(user);
}

When calling createOrUpdateUser() with an email address that already exists, that user’s name will be updated. Otherwise, a new user will be inserted into the database.

Inserts can also be used with more advanced queries. For instance, let’s say we’re building a dictionary and want to keep track of how many times we encountered a word. A table for that might look like

class Words extends Table {
  TextColumn get word => text()();
  IntColumn get usages => integer().default(const Constant(1))();

  @override
  Set<Column> get primaryKey => {word};
}

By using a custom upserts, we can insert a new word or increment its usages counter if it already exists:

Future<void> trackWord(String word) {
  return into(words).insert(
    WordsCompanion.insert(word: word),
    onConflict: DoUpdate((old) => WordsCompanion.custom(usages: old.usages + Constant(1))),
  );
}

Note that this requires a fairly recent sqlite3 version (3.24.0) that might not be available on older Android devices when using moor_flutter. moor_ffi includes the latest sqlite on Android, so consider using it if you want to support upserts.