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Migration example

Here is a simple example of a database schema migration where:

  • a column is added to an existing table
  • a table is added
// Our database path
String path;
// Our database once opened
Database db;

1st version

The first version creates a Company table with a name column.

/// Create tables
void _createTableCompanyV1(Batch batch) {
  batch.execute('DROP TABLE IF EXISTS Company');
  batch.execute('''CREATE TABLE Company (
    id INTEGER PRIMARY KEY AUTOINCREMENT,
    name TEXT
)''');
}

// First version of the database
db = await factory.openDatabase(path,
    options: OpenDatabaseOptions(
        version: 1,
        onCreate: (db, version) async {
          var batch = db.batch();
          _createTableCompanyV1(batch);
          await batch.commit();
        },
        onDowngrade: onDatabaseDowngradeDelete));

2nd version

Let say we want to add a new table Employee with a reference to a Company entity. We also want to add a new column description in the Company entity.

We handle the creation of a fresh database in onCreate and handle the schema migration in onUpgrade. Also since we want to use foreign key constraints, we configure our access in onConfigure.

/// Let's use FOREIGN KEY constraints
Future onConfigure(Database db) async {
  await db.execute('PRAGMA foreign_keys = ON');
}

/// Create Company table V2
void _createTableCompanyV2(Batch batch) {
  batch.execute('DROP TABLE IF EXISTS Company');
  batch.execute('''CREATE TABLE Company (
    id INTEGER PRIMARY KEY AUTOINCREMENT,
    name TEXT,
    description TEXT
)''');
}

/// Update Company table V1 to V2
void _updateTableCompanyV1toV2(Batch batch) {
  batch.execute('ALTER TABLE Company ADD description TEXT');
}

/// Create Employee table V2
void _createTableEmployeeV2(Batch batch) {
  batch.execute('DROP TABLE IF EXISTS Employee');
  batch.execute('''CREATE TABLE Employee (
    id INTEGER PRIMARY KEY AUTOINCREMENT,
    name TEXT,
    companyId INTEGER,
    FOREIGN KEY (companyId) REFERENCES Company(id) ON DELETE CASCADE
)''');
}

// 2nd version of the database
db = await factory.openDatabase(path,
    options: OpenDatabaseOptions(
        version: 2,
        onConfigure: onConfigure,
        onCreate: (db, version) async {
          var batch = db.batch();
          // We create all the tables
          _createTableCompanyV2(batch);
          _createTableEmployeeV2(batch);
          await batch.commit();
        },
        onUpgrade: (db, oldVersion, newVersion) async {
          var batch = db.batch();
          if (oldVersion == 1) {
            // We update existing table and create the new tables
            _updateTableCompanyV1toV2(batch);
            _createTableEmployeeV2(batch);
          }
          await batch.commit();
        },
        onDowngrade: onDatabaseDowngradeDelete));

You will have to restart your app when you change your application schema. Flutter Hot-reload won't work unless you properly close currently opened databases.

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