feat(discovery): world visit tracking endpoint with atomic play_count increment, last_played_at, and deduplication

backend/app/routers/discovery.py

@@ -0,0 +1,134 @@
+"""Discovery router - visit tracking and world browsing.
+
+docs/planning/discovery.md (tasks 42726e60fe10, 300da1dcddf3, 33706f401ad9).
+"""
+
+from datetime import datetime, timedelta, timezone
+from typing import Annotated
+
+from fastapi import APIRouter, Depends, Header, HTTPException, Query, Request, status
+from pydantic import BaseModel
+from sqlalchemy import func, select, update
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.database import get_db
+from app.dependencies import OptionalUser
+from app.models.world import World, WorldVisibility
+from app.models.world_visit import WorldVisit
+
+router = APIRouter()
+
+DbDep = Annotated[AsyncSession, Depends(get_db)]
+
+
+class VisitRequest(BaseModel):
+    anonymous_token: str | None = None
+    idempotency_key: str | None = None
+
+
+class VisitResponse(BaseModel):
+    play_count: int
+    last_played_at: datetime | None = None
+
+    model_config = {"from_attributes": True}
+
+
+DEDUP_WINDOW_SECONDS = 60
+
+
+@router.post("/worlds/{world_id}/visit", response_model=VisitResponse)
+async def record_world_visit(
+    world_id: int,
+    request: Request,
+    db: DbDep,
+    current_user: OptionalUser = None,
+    visit_data: VisitRequest | None = None,
+    x_idempotency_key: Annotated[str | None, Header(alias="X-Idempotency-Key")] = None,
+    idempotency_key: str | None = Query(None),
+):
+    """Record a visit to a world.
+
+    - Increments worlds.play_count atomically (World.play_count + 1 in UPDATE).
+    - Updates worlds.last_played_at to func.now().
+    - Records row in world_visits (with world_id, user_id nullable, visited_at).
+    - Checks visibility: 404 if world not found or not PUBLIC and not owned by user.
+    - Idempotent: repeated calls within deduplication window or with same token/key do not duplicate.
+    """
+    # 1. Fetch world
+    res = await db.execute(select(World).where(World.id == world_id))
+    world = res.scalar_one_or_none()
+    if not world:
+        raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail="World not found")
+
+    # 2. Check visibility: public or owned by requesting user
+    if world.visibility != WorldVisibility.PUBLIC:
+        if not current_user or world.owner_id != current_user.id:
+            raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail="World not found")
+
+    # Determine token / idempotency key
+    explicit_key = (
+        (visit_data.idempotency_key if visit_data else None)
+        or (visit_data.anonymous_token if visit_data else None)
+        or x_idempotency_key
+        or idempotency_key
+    )
+
+    fallback_host = request.client.host if request.client else "unknown_client"
+    dedup_key = explicit_key or (f"client_{fallback_host}" if not current_user else None)
+
+    # 3. Deduplication check
+    cutoff = datetime.now(timezone.utc) - timedelta(seconds=DEDUP_WINDOW_SECONDS)
+    is_duplicate = False
+
+    if current_user:
+        dedup_res = await db.execute(
+            select(WorldVisit).where(
+                WorldVisit.world_id == world_id,
+                WorldVisit.user_id == current_user.id,
+                WorldVisit.visited_at >= cutoff,
+            ).limit(1)
+        )
+        if dedup_res.scalar_one_or_none():
+            is_duplicate = True
+    elif dedup_key:
+        dedup_res = await db.execute(
+            select(WorldVisit).where(
+                WorldVisit.world_id == world_id,
+                WorldVisit.anonymous_token == dedup_key,
+                WorldVisit.visited_at >= cutoff,
+            ).limit(1)
+        )
+        if dedup_res.scalar_one_or_none():
+            is_duplicate = True
+
+    if is_duplicate:
+        return VisitResponse(
+            play_count=world.play_count,
+            last_played_at=world.last_played_at,
+        )
+
+    # 4. Atomic increment and visit log in single transaction
+    await db.execute(
+        update(World)
+        .where(World.id == world_id)
+        .values(
+            play_count=World.play_count + 1,
+            last_played_at=func.now(),
+        )
+    )
+
+    visit = WorldVisit(
+        world_id=world_id,
+        user_id=current_user.id if current_user else None,
+        anonymous_token=dedup_key if not current_user else None,
+    )
+    db.add(visit)
+    await db.commit()
+
+    # Re-fetch or refresh to return updated fields
+    await db.refresh(world)
+
+    return VisitResponse(
+        play_count=world.play_count,
+        last_played_at=world.last_played_at,
+    )