"""backups: record when a configuration was last confirmed unchanged.

Dedup means an unchanged config writes NO revision, so `collectedat` moves only
when something changes. A machine whose settings have been stable for six months
therefore has a six-month-old newest revision and is entirely healthy - and
ShopDB had no way to tell it apart from a machine whose backup stopped running
six months ago. The evidence that a check happened existed only in a log file on
the PC.

`lastseenat` records the check rather than the change. It is touched on every
matching post, including the no-op that writes nothing else.

Backfilled from `collectedat` (falling back to `createdat`) so existing rows
start from the last moment we can actually prove the config was current, rather
than from now - claiming a fresh check that never happened would be worse than
saying nothing.
"""
from alembic import op
import sqlalchemy as sa


# revision identifiers, used by Alembic.
revision = 'backups0002lastseenat'
down_revision = 'backups0001baseline'
branch_labels = None
depends_on = None


def upgrade():
    columns = {c['name'] for c in
               sa.inspect(op.get_bind()).get_columns('backuprevisions')}
    if 'lastseenat' not in columns:
        op.add_column('backuprevisions',
                      sa.Column('lastseenat', sa.DateTime(), nullable=True))
        op.execute('UPDATE backuprevisions '
                   'SET lastseenat = COALESCE(collectedat, createdat)')


def downgrade():
    columns = {c['name'] for c in
               sa.inspect(op.get_bind()).get_columns('backuprevisions')}
    if 'lastseenat' in columns:
        op.drop_column('backuprevisions', 'lastseenat')