refactor(discover): one shelf query per kind, one RAIL_LIMIT, a true rollup comment

The comfort shelf resolved each pinned channel with three queries (the
row, its unwatched count, its newest video) and each playlist with two.
channels_summary gets the count and the newest row per channel as window
functions over one filtered set, playlists_summary reads every pinned list
in one query, and the rows come in one IN() each — so the shelf's cost no
longer grows with its length. A test counts the statements.

RAIL_LIMIT had two definitions held in step by a comment; the Plex Discover
route now imports the domain's. And rollup's "continue the latest-started
episode" was false: it takes the last in-progress one in broadcast order,
which is also what Home's next_episode picks.

backend/app/discover/pins.py

@@ -8,14 +8,16 @@ second one.
 
 Every shelf card carries a resolved `next`: the thing that actually plays when you click the tile —
 a show's in-progress-or-next episode, a channel's newest unwatched video, a playlist's first
-unwatched item. It costs a query per non-leaf pin, which is affordable precisely because this shelf is
-short and hand-curated, and it buys the property the whole surface exists for: one click, no
-intermediate page, no second decision.
+unwatched item. That buys the property the whole surface exists for: one click, no intermediate page,
+no second decision. Channels and playlists are resolved for the whole shelf at once (`_Batch`), so the
+cost does not grow a query or two per pin.
 
 Domain module, so nothing here raises HTTP. `resolve_ref` returns None for "no such thing, or not
 yours" and the route turns that into its 404 — core never imports routes.
 """
 
+from dataclasses import dataclass, field
+
 from sqlalchemy import func, select
 from sqlalchemy.orm import Session
 
@@ -103,7 +105,34 @@ def next_position(db: Session, user: User) -> int:
     )
 
 
-def _shelf_card(db: Session, user: User, pin: ComfortPin) -> dict | None:
+@dataclass
+class _Batch:
+    """What the shelf resolves for every pinned channel and playlist in one go."""
+
+    channels: dict[str, Channel] = field(default_factory=dict)
+    channel_summary: dict[str, tuple[int, dict | None]] = field(default_factory=dict)
+    playlists: dict[int, Playlist] = field(default_factory=dict)
+    playlist_summary: dict[int, tuple[int, dict | None]] = field(default_factory=dict)
+
+    @classmethod
+    def load(cls, db: Session, user: User, pins: list[ComfortPin]) -> "_Batch":
+        ch_ids = [p.channel_id for p in pins if p.channel_id is not None]
+        pl_ids = [p.playlist_id for p in pins if p.playlist_id is not None]
+        b = cls()
+        if ch_ids:
+            b.channels = {
+                c.id: c for c in db.scalars(select(Channel).where(Channel.id.in_(ch_ids)))
+            }
+            b.channel_summary = queries.channels_summary(db, user, ch_ids)
+        if pl_ids:
+            b.playlists = {
+                p.id: p for p in db.scalars(select(Playlist).where(Playlist.id.in_(pl_ids)))
+            }
+            b.playlist_summary = queries.playlists_summary(db, user, pl_ids)
+        return b
+
+
+def _shelf_card(db: Session, user: User, pin: ComfortPin, batch: _Batch) -> dict | None:
     """One pin as a unified card with its resolved `next`. None when the pinned row has gone (the FK
     cascade removes the pin itself, so this is only the window between a delete and a refetch)."""
     card: dict | None = None
@@ -117,11 +146,11 @@ def _shelf_card(db: Session, user: User, pin: ComfortPin) -> dict | None:
         nxt = dict(card)  # a video plays itself
 
     elif pin.channel_id is not None:
-        ch = db.get(Channel, pin.channel_id)
+        ch = batch.channels.get(pin.channel_id)
         if ch is None:
             return None
-        card = cards.from_channel(ch, queries.channel_unwatched_count(db, user, ch.id))
-        row = queries.channel_next_video(db, user, ch.id)
+        count, row = batch.channel_summary.get(ch.id, (0, None))
+        card = cards.from_channel(ch, count)
         nxt = cards.from_video(row) if row else None
 
     elif pin.plex_item_id is not None:
@@ -142,10 +171,10 @@ def _shelf_card(db: Session, user: User, pin: ComfortPin) -> dict | None:
         nxt = cards.from_plex(ep) if ep else None
 
     elif pin.playlist_id is not None:
-        pl = db.get(Playlist, pin.playlist_id)
+        pl = batch.playlists.get(pin.playlist_id)
         if pl is None:
             return None
-        count, row = queries.playlist_summary(db, user, pl.id)
+        count, row = batch.playlist_summary.get(pl.id, (0, None))
         card = cards.from_playlist(pl, count, row.get("thumbnail_url") if row else None)
         nxt = cards.from_video(row) if row else None
 
@@ -162,8 +191,10 @@ def _shelf_card(db: Session, user: User, pin: ComfortPin) -> dict | None:
 def shelf(db: Session, user: User) -> list[dict]:
     """The whole shelf, in the user's own order."""
     out = []
-    for pin in list_pins(db, user):
-        card = _shelf_card(db, user, pin)
+    pins = list_pins(db, user)
+    batch = _Batch.load(db, user, pins)
+    for pin in pins:
+        card = _shelf_card(db, user, pin, batch)
         if card is not None:
             out.append(card)
     return out

backend/app/discover/queries.py

@@ -43,8 +43,9 @@ from app.plex import rollup
 from app.plex.cards import leaf_card, movie_card, show_card
 from app.plex.query import apply_show_state_filter, content_libs
 
-# How many rows a single rail may carry. Matches `routes/plex/discover.py`'s RAIL_LIMIT on purpose —
-# the two surfaces should feel like the same app, and a rail is a rail.
+# How many rows a single rail may carry — Home's rails and the Plex module's Discover rails alike
+# (`routes/plex/discover.py` imports it): the two surfaces should feel like the same app, and a rail
+# is a rail. One definition, because a comment was all that held two copies in step.
 RAIL_LIMIT = 20
 
 # The feed parameters a Home rail runs with. Spelled out rather than borrowed from the route's
@@ -129,18 +130,6 @@ def yt_surprise(db: Session, user: User, budget: str, seed: int, limit: int) ->
     return [r for r in out if not r["position_seconds"]][:limit]
 
 
-def channel_next_video(db: Session, user: User, channel_id: str) -> dict | None:
-    """The newest unwatched video of a pinned channel — what "put on that channel" resolves to."""
-    # `library_source="all"`: a pinned channel means the whole channel, including videos that only
-    # ever arrived through a search. The feed's "organic" default is a feed-scoping choice, not a
-    # statement about what belongs to a channel.
-    params = dict(_FEED_BASE, channel_ids=[channel_id], scope="all", library_source="all")
-    query, rank_expr = filtered_query(db, user, **params)
-    keys = sort_keys("newest", 0, rank_expr)
-    row = db.execute(query.order_by(*order_by(keys)).limit(1)).first()
-    return serialize(row) if row is not None else None
-
-
 def yt_video_row(db: Session, user: User, video_id: str) -> dict | None:
     """One video as a feed row, with this user's watch state.
 
@@ -158,31 +147,55 @@ def yt_video_row(db: Session, user: User, video_id: str) -> dict | None:
     return serialize(row) if row is not None else None
 
 
-def channel_unwatched_count(db: Session, user: User, channel_id: str) -> int:
-    """How many unwatched videos a pinned channel is holding — the tile's only honest promise, since a
-    channel has no runtime of its own."""
-    params = dict(_FEED_BASE, channel_ids=[channel_id], scope="all", library_source="all")
-    query, _ = filtered_query(db, user, **params)
-    return int(db.execute(select(func.count()).select_from(query.subquery())).scalar() or 0)
+def channels_summary(
+    db: Session, user: User, channel_ids: list[str]
+) -> dict[str, tuple[int, dict | None]]:
+    """For each pinned channel: (unwatched count, its newest unwatched video) — the tile's only honest
+    promise, since a channel has no runtime of its own, and what "put on that channel" resolves to.
+
+    ONE query for the whole shelf, not two per channel: the count and the first row per channel are
+    window functions over the same filtered set. `library_source="all"`: a pinned channel means the
+    whole channel, including videos that only ever arrived through a search — the feed's "organic"
+    default is a feed-scoping choice, not a statement about what belongs to a channel."""
+    if not channel_ids:
+        return {}
+    params = dict(_FEED_BASE, channel_ids=list(channel_ids), scope="all", library_source="all")
+    query, rank_expr = filtered_query(db, user, **params)
+    newest = order_by(sort_keys("newest", 0, rank_expr))
+    per_channel = {"partition_by": Video.channel_id}
+    sub = query.add_columns(
+        func.row_number().over(**per_channel, order_by=newest).label("_rn"),
+        func.count().over(**per_channel).label("_total"),
+    ).subquery()
+    rows = db.execute(select(sub).where(sub.c._rn == 1)).all()
+    return {r.channel_id: (int(r._total), serialize(r)) for r in rows}
 
 
-def playlist_summary(db: Session, user: User, playlist_id: int) -> tuple[int, dict | None]:
-    """(item count, the row to play next) for a pinned playlist — its first not-yet-watched item, or
-    its first item once the whole list is watched. The same row doubles as the tile's artwork, so a
-    shelf of playlists is scannable instead of a row of blank boxes."""
+def playlists_summary(
+    db: Session, user: User, playlist_ids: list[int]
+) -> dict[int, tuple[int, dict | None]]:
+    """For each pinned playlist: (item count, the row to play next) — its first not-yet-watched item,
+    or its first item once the whole list is watched. The same row doubles as the tile's artwork, so
+    a shelf of playlists is scannable instead of a row of blank boxes. One query for the whole shelf."""
+    if not playlist_ids:
+        return {}
     st = aliased(VideoState)
     rows = db.execute(
-        select(*feed_columns(user, st), PlaylistItem.position)
+        select(*feed_columns(user, st), PlaylistItem.playlist_id.label("_playlist"))
         .join(PlaylistItem, PlaylistItem.video_id == Video.id)
         .join(Channel, Channel.id == Video.channel_id)
         .outerjoin(st, and_(st.video_id == Video.id, st.user_id == user.id))
-        .where(PlaylistItem.playlist_id == playlist_id)
-        .order_by(PlaylistItem.position.asc())
+        .where(PlaylistItem.playlist_id.in_(playlist_ids))
+        .order_by(PlaylistItem.playlist_id, PlaylistItem.position.asc())
     ).all()
-    if not rows:
-        return 0, None
-    nxt = next((r for r in rows if (r.status or "new") not in ("watched", "hidden")), rows[0])
-    return len(rows), serialize(nxt)
+    grouped: dict[int, list] = {}
+    for r in rows:
+        grouped.setdefault(r._playlist, []).append(r)
+    out: dict[int, tuple[int, dict | None]] = {}
+    for pid, items in grouped.items():
+        nxt = next((r for r in items if (r.status or "new") not in ("watched", "hidden")), items[0])
+        out[pid] = (len(items), serialize(nxt))
+    return out
 
 
 # ── Plex ──────────────────────────────────────────────────────────────────────────────────────────

backend/app/plex/rollup.py

@@ -43,7 +43,9 @@ def rollup(cards: list[dict]) -> dict:
     ]
     resume = None
     if inprog:
-        resume = inprog[-1]  # continue the latest-started episode
+        # The LAST in-progress episode in broadcast order — no timestamp is consulted. Home's
+        # `discover.queries.next_episode` picks the same one, so both Resume buttons agree.
+        resume = inprog[-1]
     else:
         resume = next((c for c in cards if c.get("status") != "watched"), None)
     return {

backend/app/routes/plex/discover.py

@@ -32,12 +32,12 @@ from app.plex.query import (
     meta_filter_conds,
     top_art_keys,
 )
+from app.discover.queries import RAIL_LIMIT
 from app.plex.tmdb import resolve_recs
 from app.routes.plex._shared import _norm_lang
 
 router = APIRouter(prefix="/api/plex", tags=["plex"])
 
-RAIL_LIMIT = 20
 GENRE_TILES = 12
 SURPRISE_PER_KIND = 3  # Surprise-me: at most this many movies + this many shows (6 total)
 

backend/tests/test_discover.py

@@ -544,6 +544,73 @@ def test_shelf_resolves_a_playlist_to_its_first_unwatched_item(db):
     assert card["next"]["id"] == "vmed"
 
 
+def _count_statements(db, fn) -> int:
+    """How many SQL statements `fn` sends through the session's connection."""
+    from sqlalchemy import event
+
+    seen: list[str] = []
+    engine = db.get_bind()
+    listener = lambda *a, **k: seen.append(a[2])  # noqa: E731 — (conn, cursor, statement, …)
+    event.listen(engine, "before_cursor_execute", listener)
+    try:
+        fn()
+    finally:
+        event.remove(engine, "before_cursor_execute", listener)
+    return len(seen)
+
+
+def test_the_shelf_resolves_each_channel_and_playlist_on_its_own_terms(db):
+    """Batched, the per-channel count and newest video come from one windowed query: each channel
+    must still get ITS count and ITS newest, not the shelf's overall first row."""
+    user, ch = _seed_yt(db)
+    other = Channel(id="UCother", title="Other")
+    db.add(other)
+    db.flush()
+    db.add(Video(id="vother", channel_id=other.id, title="x", published_at=NOW - timedelta(days=9)))
+    pl = Playlist(user_id=user.id, name="Két elem")
+    empty = Playlist(user_id=user.id, name="Üres")
+    db.add_all([pl, empty])
+    db.flush()
+    db.add(PlaylistItem(playlist_id=pl.id, video_id="vlong", position=0))
+    for i, target in enumerate(
+        [
+            {"channel_id": ch.id},
+            {"channel_id": other.id},
+            {"playlist_id": pl.id},
+            {"playlist_id": empty.id},
+        ]
+    ):
+        db.add(ComfortPin(user_id=user.id, position=i, **target))
+    db.commit()
+
+    got = [
+        (c.get("unwatched_count", c.get("item_count")), (c["next"] or {}).get("id"))
+        for c in pins.shelf(db, user)
+    ]
+    assert got == [(3, "vshort"), (1, "vother"), (1, "vlong"), (0, None)]
+
+
+def test_the_shelf_costs_the_same_queries_for_one_pin_or_many(db):
+    """A channel pin used to cost three queries and a playlist pin two, each; the shelf now
+    resolves every channel and every playlist together."""
+    user, ch = _seed_yt(db)
+    extra = [Channel(id=f"UCx{i}", title=f"x{i}") for i in range(3)]
+    lists = [Playlist(user_id=user.id, name=f"p{i}") for i in range(4)]
+    db.add_all(extra + lists)
+    db.flush()
+    db.add(ComfortPin(user_id=user.id, channel_id=ch.id, position=0))
+    db.add(ComfortPin(user_id=user.id, playlist_id=lists[0].id, position=1))
+    db.commit()
+    few = _count_statements(db, lambda: pins.shelf(db, user))
+
+    for i, c in enumerate(extra):
+        db.add(ComfortPin(user_id=user.id, channel_id=c.id, position=10 + i))
+    for i, p in enumerate(lists[1:]):
+        db.add(ComfortPin(user_id=user.id, playlist_id=p.id, position=20 + i))
+    db.commit()
+    assert _count_statements(db, lambda: pins.shelf(db, user)) == few
+
+
 def test_a_pinned_video_survives_leaving_the_feed_scope(db):
     """The shelf is the fallback surface. An entry silently vanishing because the user unsubscribed
     from the channel is exactly the failure it must not have."""