"""Enforce NOT NULL on server-defaulted timestamps the models already require.

0011, 0012, 0014 and 0017 created these columns with ``server_default=now()``
but without ``nullable=False``, so the database accepted NULL while the models
declare them non-optional. No application path writes NULL to them (inserts
omit them and rely on the default; updates set ``now``), and before this
revision was written no NULL existed in production or in the development
database.

The upgrade refuses to run — changing nothing — if any NULL is found, instead
of guessing a value for it.

Revision ID: 0020_not_null_timestamps
Revises: 0019_fix_timestamp_defaults
"""
from alembic import op
import sqlalchemy as sa

revision = "0020_not_null_timestamps"
down_revision = "0019_fix_timestamp_defaults"
branch_labels = None
depends_on = None

COLUMNS = (
    ("admin_users", "created_at"),
    ("agent_events", "occurred_at"),
    ("agent_snapshots", "updated_at"),
    ("ai_eval_cache", "created_at"),
    ("ai_runs", "started_at"),
    ("ai_work_items", "created_at"),
    ("ai_work_items", "updated_at"),
    ("audit_events", "occurred_at"),
    ("fiscal_context_entries", "created_at"),
    ("fiscal_context_entries", "updated_at"),
    ("installations", "created_at"),
    ("installations", "updated_at"),
    ("kb_knowledge", "created_at"),
    ("kb_observations", "observed_at"),
    ("kb_products", "created_at"),
    ("kb_products", "updated_at"),
    ("tenant_product_context", "created_at"),
    ("tenant_product_context", "updated_at"),
    ("web_sessions", "created_at"),
    ("web_sessions", "last_seen_at"),
)


def upgrade():
    bind = op.get_bind()
    offenders = []
    for table, column in COLUMNS:
        nulls = bind.execute(sa.text(f'SELECT count(*) FROM "{table}" WHERE "{column}" IS NULL')).scalar()
        if nulls:
            offenders.append(f"{table}.{column}={nulls}")
    if offenders:
        raise RuntimeError("0020 aborted, NULL values present (fix them explicitly first): " + ", ".join(offenders))
    for table, column in COLUMNS:
        op.alter_column(table, column, nullable=False, existing_type=sa.DateTime(timezone=True), existing_server_default=sa.func.now())


def downgrade():
    for table, column in reversed(COLUMNS):
        op.alter_column(table, column, nullable=True, existing_type=sa.DateTime(timezone=True), existing_server_default=sa.func.now())