Identify and fix N+1 query anti-patterns in Ecto/Phoenix applications.
\nEnum.mapjoin + preload when filtering by association# BAD: N+1 queries\nusers\n|> Enum.map(fn user -> Repo.get(Order, user.order_id) end)\n\n# GOOD: Single query with preload\nusers\n|> Repo.preload(:orders)\n# BAD: Lazy loading triggers N queries\nfor user <- users do\n user.posts # Triggers query for each user!\nend\n\n# GOOD: Eager load first\nusers = Repo.all(User) |> Repo.preload(:posts)\nfor user <- users do\n user.posts # Already loaded\nend\n# BAD: N+1 for nested associations\nuser.posts |> Enum.map(fn post -> post.comments end)\n\n# GOOD: Nested preload\nRepo.preload(user, posts: :comments)\nUse Grep with context lines (-B 5 -A 5) to find Enum.map near Repo. calls in lib/**/*.ex.\nUse Grep to find association access patterns (.posts, .comments, .orders) in lib/**/*.ex.\nUse Grep with context (-B 3) to find Repo.get or Repo.one near loop patterns (for, Enum) in lib/**/*.ex.
Use Grep to find all Repo. calls in a context module, then verify each query has appropriate preloads.
For detailed patterns, see:
\n${CLAUDE_SKILL_DIR}/references/preload-patterns.md - Efficient preloading strategies${CLAUDE_SKILL_DIR}/references/query-optimization.md - Query batching techniques