I migrate from FMDB to GRDB and one thing I'm stuck a little is how it's better to mig

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GRDB migrator and previous FMDB database about grdb.swift HOT 3 CLOSED

zaplitny commented on June 2, 2024

GRDB migrator and previous FMDB database

from grdb.swift.

Comments (3)

groue commented on June 2, 2024

Hello @zaplitny,

It looks like this could work, yes. But since grdb_migrations is an internal table, not even mentioned in the documentation, I would not recommend messing with it directly. Future GRDB versions may modify the grdb_migrations schema, and your code would stop working.

Instead, you can rely of the fact that GRDB never modifies user_version by itself. So if it is not zero, you know that the app opens a database that was previously handled by FMDB.

You can thus register one GRDB migration per FMDB migration:

var migrator = ...

migrator.registerMigration("fmdb-1") { db in
    guard try db.userVersion() < 1 else {
        // Migration already applied by FMDB
        return
    }

    // apply script 1
}

migrator.registerMigration("fmdb-2") { db in
    guard try db.userVersion() < 2 else {
        // Migration already applied by FMDB
        return
    }
    
    // apply script 2
}

...

migrator.registerMigration("first-post-fmdb-migration") { db in
    ...
}

Also possible: just register one GRDB migration for all FMDB migrations:

var migrator = ...

migrator.registerMigration("fmdb") { db in
    let version = try db.userVersion()
    
    if version < 1 {
        // apply script 1
    }
    
    if version < 2 {
        // apply script 2
    }

    ...
}

migrator.registerMigration("first-post-fmdb-migration") { db in
    ...
}

Both sample codes use this little Database extension:

extension Database {
    func userVersion() -> Int {
        // Force-unwrap is ok: user_version is always returned.
        try Int.fetchOne(db, sql: "PRAGMA user_version")!
    }
}