"""An open grouping — a synthetic post that a later drop can still join.

Milestone 388, step E3. E2's synthetic post was sealed at creation: a creator
who added two more variants the next day started a second post. These two
columns let the group stay open and absorb the follow-up, without the post
either freezing or thrashing the feed.

## Why openness is derived rather than stored

There is no `closed_at` here on purpose. A group is open if it grew (or
started) within `ml_settings.discord_group_close_after_hours`, so openness is a
comparison rather than a state — which means lowering the setting closes old
groups and raising it reopens them, with nothing to repair either way. A stored
flag would need its own sweep to set it and its own repair path to ever change
the policy, for no gain.

## Why `resurfaced_at` is separate from `last_grew_at`

They answer different questions. `last_grew_at` is when the group last
absorbed something — it decides how long the group stays joinable and is what
the card shows. `resurfaced_at` is the FEED POSITION, advanced only when the
anti-thrash rule fires, so a group that gains one image a day updates in place
while a genuine second wave moves once. Folding them together would make every
addition a bump, which is the annoyance this step exists to avoid.

Both are NULL on every ordinary post, so the feed's sort key can COALESCE
through `resurfaced_at` without moving anything that is not a grouping.

Revision ID: 0093
Revises: 0092
Create Date: 2026-09-10

"""
from typing import Sequence, Union

import sqlalchemy as sa
from alembic import op

revision: str = "0093"
down_revision: Union[str, None] = "0092"
branch_labels: Union[str, Sequence[str], None] = None
depends_on: Union[str, Sequence[str], None] = None


def upgrade() -> None:
    op.add_column(
        "post", sa.Column("last_grew_at", sa.DateTime(timezone=True), nullable=True),
    )
    op.add_column(
        "post", sa.Column("resurfaced_at", sa.DateTime(timezone=True), nullable=True),
    )
    # No index on either. The feed already sorts on an unindexed
    # COALESCE(post_date, downloaded_at) expression, so adding resurfaced_at to
    # that COALESCE changes nothing about how the query plans — and inventing a
    # functional index here would be guessing at the fix for a cost nobody has
    # measured. Measuring it is step B2's job.

    op.add_column(
        "ml_settings",
        sa.Column(
            "discord_group_close_after_hours", sa.Float(),
            server_default=sa.text("168"), nullable=False,
        ),
    )
    op.add_column(
        "ml_settings",
        sa.Column(
            "discord_group_resurface_min_images", sa.Integer(),
            server_default="2", nullable=False,
        ),
    )
    op.add_column(
        "ml_settings",
        sa.Column(
            "discord_group_resurface_cooldown_hours", sa.Float(),
            server_default=sa.text("24"), nullable=False,
        ),
    )


def downgrade() -> None:
    op.drop_column("ml_settings", "discord_group_resurface_cooldown_hours")
    op.drop_column("ml_settings", "discord_group_resurface_min_images")
    op.drop_column("ml_settings", "discord_group_close_after_hours")
    op.drop_column("post", "resurfaced_at")
    op.drop_column("post", "last_grew_at")