Ecto's `preload` with joins uses a single query!

TIL that when you explicitly join associations in Ecto and use preload, it executes just ONE query:

# ✅ Single query - uses the joined data!
from(rt in RaceTeam,
  left_join: r in assoc(rt, :race),
  left_join: cl in assoc(rt, :clues),
  where: rt.id == ^team_id,
  preload: [race: r, clues: cl]
)
|> Repo.one()

I always thought preload meant multiple queries, but that’s only true without explicit joins:

# ❌ Multiple queries
RaceTeam
|> preload([:race, :clues])
|> Repo.one()

The joined version handles the SQL row duplication problem automatically, grouping everything into the proper nested structure. No more N+1 queries AND no manual grouping needed! 🚀