• Component used: androidx.room
• Version used: 2.6.1

It would be very helpful if AutoMigrationSpec had an onPreMigrate hook.

Example use case:

I am modifying FooEntity to add an FK constraint to BarEntity.

This is easy to do with an AutoMigration, Room handles migrating the data for you.

However, it's possible the database may contain data that does not meet this constraint (adding the constraint is intended to prevent this in the future).

If this data is not deleted the migration will fail (and fail on every attempt).

If an onPreMigrate hook existed I could use this to find and delete any FooEntity that don't meet the constraint.

Alternatives

At the moment you either have to:

• Implement the migration as a single Migration, and write all the code yourself.
• Split the migration across two different database versions that will be deployed in a single app version update. The first is a Migration that does the deletion, the second is an AutoMigration where Room generates the code to add the constraint.