Drift files

Learn everything about the new .drift files which can contain tables and queries

Drift files are a new feature that lets you write all your database code in SQL - drift will generate typesafe APIs for them.

Getting started

To use this feature, lets create two files: database.dart and tables.drift. The Dart file only contains the minimum code to setup the database:

import 'package:drift/drift.dart';
import 'package:drift/native.dart';

part 'database.g.dart';

@DriftDatabase(
  include: {'tables.drift'},
)
class MyDb extends _$MyDb {
  // This example creates a simple in-memory database (without actual
  // persistence).
  // To store data, see the database setups from other "Getting started" guides.
  MyDb() : super(NativeDatabase.memory());

  @override
  int get schemaVersion => 1;
}

We can now declare tables and queries in the drift file:

CREATE TABLE todos (
    id INT NOT NULL PRIMARY KEY AUTOINCREMENT,
    title TEXT NOT NULL,
    content TEXT NOT NULL,
    category INTEGER REFERENCES categories(id)
);

CREATE TABLE categories (
    id INT NOT NULL PRIMARY KEY AUTOINCREMENT,
    description TEXT NOT NULL
) AS Category; -- the AS xyz after the table defines the data class name

-- You can also create an index or triggers with drift files
CREATE INDEX categories_description ON categories(description);

-- we can put named sql queries in here as well:
createEntry: INSERT INTO todos (title, content) VALUES (:title, :content);
deleteById: DELETE FROM todos WHERE id = :id;
allTodos: SELECT * FROM todos;

After running the build runner with flutter pub run build_runner build, drift will write the database.g.dart file which contains the _$MyDb superclass. Let's take a look at what we got:

  • Generated data classes (Todo and Category), and companion versions for inserts (see Dart Interop for info). By default, drift strips a trailing "s" from the table name for the class. That's why we used AS Category on the second table - it would have been called Categorie otherwise.
  • Methods to run the queries:
    • a Future<int> createEntry(String title, String content) method. It creates a new todo entry with the provided data and returns the id of the entry created.
    • Future<int> deleteById(int id): Deletes a todo entry by its id, and returns the amount of rows affected.
    • Selectable<AllTodosResult> allTodos(). It can be used to get, or watch, all todo entries. It can be used with allTodos().get() and allTodos().watch().
  • Classes for select statements that don't match a table. In the example above, thats the AllTodosResult class, which contains all fields from todos and the description of the associated category.

Variables

Inside of named queries, you can use variables just like you would expect with sql. We support regular variables (?), explicitly indexed variables (?123) and colon-named variables (:id). We don't support variables declared with @ or $. The compiler will attempt to infer the variable's type by looking at its context. This lets drift generate typesafe apis for your queries, the variables will be written as parameters to your method.

When it's ambiguous, the analyzer might be unable to resolve the type of a variable. For those scenarios, you can also denote the explicit type of a variable:

myQuery(:variable AS TEXT): SELECT :variable;

In addition to the base type, you can also declare that the type is nullable:

myNullableQuery(:variable AS TEXT OR NULL): SELECT :variable;

Finally, you can declare that a variable should be required in Dart when using named parameters. To do so, add a REQUIRED keyword:

myRequiredQuery(REQUIRED :variable AS TEXT OR NULL): SELECT :variable;

Note that this only has an effect when the named_parameters build option is enabled. Further, non-nullable variables are required by default.

Arrays

If you want to check whether a value is in an array of values, you can use IN ?. That's not valid sql, but drift will desugar that at runtime. So, for this query:

entriesWithId: SELECT * FROM todos WHERE id IN ?;

Drift will generate a Selectable<Todo> entriesWithId(List<int> ids) method. Running entriesWithId([1,2]) would generate SELECT * ... id IN (?1, ?2) and bind the arguments accordingly. To make sure this works as expected, drift imposes two small restrictions:

  1. No explicit variables: WHERE id IN ?2 will be rejected at build time. As the variable is expanded, giving it a single index is invalid.
  2. No higher explicit index after a variable: Running WHERE id IN ? OR title = ?2 will also be rejected. Expanding the variable can clash with the explicit index, which is why drift forbids it. Of course, id IN ? OR title = ? will work as expected.

Defining tables

In .drift files, you can define table with a CREATE TABLE statement, just like you would write it in SQL.

Supported column types

Just like sqlite itself, we use this algorithm to determine the column type based on the declared type name.

Additionally, columns that have the type name BOOLEAN or DATETIME will have bool or DateTime as their Dart counterpart. Booleans are stored as INTEGER (either 0 or 1). Datetimes are stored as unix timestamps (INTEGER) or ISO-8601 (TEXT) depending on a configurable build option. Dart enums can automatically be stored by their index by using an ENUM() type referencing the Dart enum class:

enum Status {
   none,
   running,
   stopped,
   paused
}
import 'status.dart';

CREATE TABLE tasks (
  id INTEGER NOT NULL PRIMARY KEY,
  status ENUM(Status)
);

More information on storing enums is available in the page on type converters.

For details on all supported types, and information on how to switch between the datetime modes, see this section.

Drift-specific features

To help support drift's Dart API, CREATE TABLE statements in drift files can use special syntax for Dart-specific features. Of course, drift will strip this special syntax from the CREATE TABLE statement before it runs it.

  • You can define a custom row class for a table by appending WITH YourDartClass to a CREATE TABLE statement.
  • Alternatively, you may use AS DesiredRowClassName to change the name of the row class generated by drift.
  • In a column definition, MAPPED BY can be used to apply a converter to that column.
  • Similarly, a JSON KEY constraint can be used to define the key drift will use when serializing a row of that table to JSON.
  • Finally, AS getterName can be used as a column constraint to override the generated name of that column in Dart. This can be useful when the default column name, inspired by the name of the column in SQL, conflicts with another member of the generated table class.

Imports

You can put import statements at the top of a drift file:

import 'tables.drift'; -- single quotes are required for imports

All tables reachable from the other file will then also be visible in the current file and to the database that includes it. If you want to declare queries on tables that were defined in another drift file, you also need to import that file for the tables to be visible. Note that imports in drift file are always transitive, so in the above example you would have all imports declared in other.drift available as well. There is no export mechanism for drift files.

Importing Dart files into a drift file will also work - then, all the tables declared via Dart tables can be used inside queries. We support both relative imports and the package: imports you know from Dart.

Nested results

Many queries fetch all columns from some table, typically by using the SELECT table.* syntax. That approach can become a bit tedious when applied over multiple tables from a join, as shown in this example:

CREATE TABLE coordinates (
  id INTEGER NOT NULL PRIMARY KEY,
  lat REAL NOT NULL,
  long REAL NOT NULL
);

CREATE TABLE saved_routes (
  id INTEGER NOT NULL PRIMARY KEY,
  name TEXT NOT NULL,
  "from" INTEGER NOT NULL REFERENCES coordinates (id),
  "to" INTEGER NOT NULL REFERENCES coordinates (id)
);

routesWithPoints: SELECT r.id, r.name, f.*, t.* FROM saved_routes r
  INNER JOIN coordinates f ON f.id = r."from"
  INNER JOIN coordinates t ON t.id = r."to";

To match the returned column names while avoiding name clashes in Dart, drift will generate a class having an id, name, id1, lat, long, lat1 and a long1 field. Of course, that's not helpful at all - was lat1 coming from from or to again? Let's rewrite the query, this time using nested results:

routesWithNestedPoints: SELECT r.id, r.name, f.** AS "from", t.** AS "to" FROM saved_routes r
  INNER JOIN coordinates f ON f.id = r."from"
  INNER JOIN coordinates t ON t.id = r."to";

As you can see, we can nest a result simply by using the drift-specific table.** syntax. For this query, drift will generate the following class:

class RoutesWithNestedPointsResult {
  final int id;
  final String name;
  final Point from;
  final Point to;
  // ...
}

Great! This class matches our intent much better than the flat result class from before.

At the moment, there are some limitations with this approach:

  • ** is not yet supported in compound select statements
  • you can only use table.** if table is an actual table or a reference to it. In particular, it doesn't work for result sets from WITH clauses or table- valued functions.

You might be wondering how ** works under the hood, since it's not valid sql. At build time, drift's generator will transform ** into a list of all columns from the referred table. For instance, if we had a table foo with an id INT and a bar TEXT column. Then, SELECT foo.** FROM foo might be desugared to SELECT foo.id AS "nested_0.id", foo.bar AS "nested_0".bar FROM foo.

LIST subqueries

Starting from Drift version 1.4.0, subqueries can also be selected as a full list. Simply put the subquery in a LIST() function to include all rows of the subquery in the result set.

Re-using the coordinates and saved_routes tables introduced in the example for nested results, we add a new table storing coordinates along a route:

CREATE TABLE route_points (
  route INTEGER NOT NULL REFERENCES saved_routes (id),
  point INTEGER NOT NULL REFERENCES coordinates (id),
  index_on_route INTEGER,
  PRIMARY KEY (route, point)
);

Now, assume we wanted to query a route with information about all points along the way. While this requires two SQL statements, we can write this as a single drift query that is then split into the two statements automatically:

routeWithPoints: SELECT
    route.**,
    LIST(SELECT coordinates.* FROM route_points
      INNER JOIN coordinates ON id = point
      WHERE route = route.id
      ORDER BY index_on_route
    ) AS points
  FROM saved_routes route;

This will generate a result set containing a SavedRoute route field along with a List<Point> points list of all points along the route.

Internally, drift will split this query into two separate queries:

  • The outer SELECT route.** FROM saved_routes route SQL queries
  • A separate SELECT coordinates.* FROM route_points ... ORDER BY index_on_route query that is run for each row in the outer query. The route.id reference in the inner query is replaced with a variable that drift binds to the actual value in the outer query.

While LIST() subqueries are a very powerful feature, they can be costly when the outer query has lots of rows (as the inner query is executed for each outer row).

Dart interop

Drift files work perfectly together with drift's existing Dart API:

  • you can write Dart queries for tables declared in a drift file:
Future<void> insert(TodosCompanion companion) async {
  await into(todos).insert(companion);
}
  • by importing Dart files into a drift file, you can write sql queries for tables declared in Dart.
  • generated methods for queries can be used in transactions, they work together with auto-updating queries, etc.

If you're using the fromJson and toJson methods in the generated Dart classes and need to change the name of a column in json, you can do that with the JSON KEY column constraints, so id INT NOT NULL JSON KEY userId would generate a column serialized as "userId" in json.

Dart components in SQL

You can make most of both SQL and Dart with "Dart Templates", which is a Dart expression that gets inlined to a query at runtime. To use them, declare a $-variable in a query:

_filterTodos: SELECT * FROM todos WHERE $predicate;

Drift will generate a Selectable<Todo> _filterTodos(Expression<bool> predicate) method that can be used to construct dynamic filters at runtime:

Stream<List<Todo>> watchInCategory(int category) {
    return _filterTodos(todos.category.equals(category)).watch();
}

This lets you write a single SQL query and dynamically apply a predicate at runtime! This feature works for

  • expressions, as you've seen in the example above
  • single ordering terms: SELECT * FROM todos ORDER BY $term, id ASC will generate a method taking an OrderingTerm.
  • whole order-by clauses: SELECT * FROM todos ORDER BY $order
  • limit clauses: SELECT * FROM todos LIMIT $limit
  • insertables for insert statements: INSERT INTO todos $row generates an Insertable<TodoEntry> row parameter

When used as expression, you can also supply a default value in your query:

_filterTodos ($predicate = TRUE): SELECT * FROM todos WHERE $predicate;

This will make the predicate parameter optional in Dart. It will use the default SQL value (here, TRUE) when not explicitly set.

Type converters

You can import and use type converters written in Dart in a drift file. Importing a Dart file works with a regular import statement. To apply a type converter on a column definition, you can use the MAPPED BY column constraints:

CREATE TABLE users (
  id INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT,
  name TEXT,
  preferences TEXT MAPPED BY `const PreferenceConverter()`
);

More details on type converts in drift files are available here.

When using type converters, we recommend the apply_converters_on_variables build option. This will also apply the converter from Dart to SQL, for instance if used on variables: SELECT * FROM users WHERE preferences = ?. With that option, the variable will be inferred to Preferences instead of String.

Existing row classes

You can use custom row classes instead of having drift generate one for you. For instance, let's say you had a Dart class defined as

class User {
 final int id;
 final String name;

 User(this.id, this.name);
}

Then, you can instruct drift to use that class as a row class as follows:

import 'row_class.dart'; --import for where the row class is defined

CREATE TABLE users (
  id INTEGER NOT NULL PRIMARY KEY,
  name TEXT NOT NULL,
) WITH User; -- This tells drift to use the existing Dart class

When using custom row classes defined in another Dart file, you also need to import that file into the file where you define the database. For more general information on this feature, please check this page.

Result class names

For most queries, drift generates a new class to hold the result. This class is named after the query with a Result suffix, e.g. a myQuery query would get a MyQueryResult class.

You can change the name of a result class like this:

routesWithNestedPoints AS FullRoute: SELECT r.id, -- ...

This way, multiple queries can also share a single result class. As long as they have an identical result set, you can assign the same custom name to them and drift will only generate one class.

For queries that select all columns from a table and nothing more, drift won't generate a new class and instead re-use the dataclass that it generates either way. Similarly, for queries with only one column, drift will just return that column directly instead of wrapping it in a result class. It's not possible to override this behavior at the moment, so you can't customize the result class name of a query if it has a matching table or only has one column.

Supported statements

At the moment, the following statements can appear in a .drift file.

  • import 'other.drift': Import all tables and queries declared in the other file into the current file.
  • DDL statements: You can put CREATE TABLE, CREATE VIEW, CREATE INDEX and CREATE TRIGGER statements into drift files.
  • Query statements: We support INSERT, SELECT, UPDATE and DELETE statements.

All imports must come before DDL statements, and those must come before named queries.

If you need support for another statement, or if drift rejects a query you think is valid, please create an issue!