This migration[0] adds a CHECK constraint to a column.
When the migration is started, a new column with the constraint is created and values from the old column are backfilled using the 'up' SQL from the migration. The 'up' SQL rewrites values that don't meet the constraint so that they do.
The same 'up' SQL is used 'on the fly' as data is written to the old schema by applications - the 'up' SQL is used (as part of a trigger) to copy data into the new column, rewriting as necessary to ensure the constraint on the new column is met.
As the sibling comment makes clear, it is currently the migration author's responsibility to ensure that the 'up' SQL really does rewrite values so that they meet the constraint.
If I may be so free, I read your docs quite carefully last night and it wasn’t immediately clear to me that “up” and “down” are automagically run to keep both the old and the new data in sync, while a migration is pending. To my reading this is a spectacular feature because it solves the “the only way is forward” problem (where you can’t actually ever roll back a migration or risk losing / making inconsistent some recent data) for many kinds of migrations.
May I suggest spelling this out more explicitly in the docs? As far as I can tell it’s only mentioned by example halfway down the tutorial. To me it seems like one of pgroll’s true killer features, worth more coverage :-)
Yes, this is the case as of today. It's important to get the up/down functions right.
The good thing is that this will probably be detected during the start phase, as data would fail to be backfilled.
We are thinking of ideas to improve this in the longer term, for example:
* Having a dry-run mode where you check that the data present in the DB will stay valid after applying the up function
* Optionally allow for this to happen, but "quarantine" the affected rows so they are no longer available in the new schema.
