Ecto Patterns

What it does

When your agent uses it

• A task touches Ecto
• Even a how-to question: schemas

Example prompts

• “/ecto-patterns”

Workflow steps

9 steps, taken from the first numbered list in SKILL.md.

1. 1CHANGESETS ARE FOR EXTERNAL DATA — Use cast/4 for user/API input, change/2 or put_change/3 for internal trusted data
2. 2NEVER USE :float FOR MONEY — Always use :decimal or :integer (cents)
3. 3NO RAILS-STYLE POLYMORPHIC ASSOCIATIONS — They break foreign key constraints; use multiple nullable FKs or separate join tables
4. 4ALWAYS PIN VALUES IN QUERIES — u.name == ^user_input is safe, string interpolation causes SQL injection
5. 5PRELOAD COLLECTIONS, NOT INDIVIDUALS — Preloading in loops = N+1 queries
6. 6CONSTRAINTS BEAT VALIDATIONS FOR RACE CONDITIONS — Validations provide quick feedback, constraints provide DB-level safety
7. 7SEPARATE QUERIES FOR has_many, JOIN FOR belongs_to — Avoids row multiplication
8. 8NO IMPLICIT CROSS JOINS — from(a in A, b in B) without on: creates Cartesian product
9. 9DEDUP BEFORE cast_assoc WITH SHARED DATA — When multiple parents share child data, deduplicate child records BEFORE building changesets…

What it can do on your machine

Read from SKILL.md and the folder at commit 9767a82. It shows what the files ask for, not the result of running them.

• Tool permissions

  Pre-approves nothing: there is no allowed-tools line, so your agent's usual permission prompts apply.

  From allowed-tools in the SKILL.md frontmatter.

• Runs code

  No scripts in the folder and no shell commands in SKILL.md (its code samples are elixir).

  From the folder's file list and the shell code blocks in SKILL.md.

• Network

  No URLs in SKILL.md.

  From URLs in SKILL.md, links to its own repository left out.

• Credentials

  Names no API keys, tokens, secrets or passwords.

  From names ending in _API_KEY, _TOKEN, _SECRET, _KEY or _PASSWORD in SKILL.md.

Context cost

Ecto Patterns loads about 859 tokens when it runs, and up to ~6.9k if it reads all its reference files. Until then it costs about 54 tokens; SKILL.md has 282 words of instructions outside code blocks.

Always · name and description, kept in context so the agent knows when to use it

~54

When it runs · the whole SKILL.md, loaded when a task matches

~859

With references · SKILL.md plus every file in references/, read only if the agent opens them

~6.9k

Estimates: characters ÷ 4, the usual rule of thumb; real counts depend on the model's tokenizer. Scripts and assets cost tokens only if the agent reads them.

Safety

Automated static check — not a guarantee. Review scripts before installing. It scans the text of SKILL.md for risky patterns (piping downloads into a shell, reading credential files, hidden Unicode, destructive commands); files beside SKILL.md are not scanned.

SKILL.md

The full file from oliver-kriska/claude-elixir-phoenix at commit 9767a82, republished under its MIT licence (© oliver-kriska). 282 words, ~859 tokens.

name

ecto-patterns

description

Use when a task touches Ecto, even a how-to question: schemas, changesets, validations, queries, preloads, Multi, migrations, constraints, money fields. Load it before writing Ecto code. Skip for Ash.

defmodule MyApp.Context.Entity do
  use Ecto.Schema
  import Ecto.Changeset

  @primary_key {:id, :binary_id, autogenerate: true}
  @foreign_key_type :binary_id

  schema "entities" do
    field :name, :string
    field :status, Ecto.Enum, values: [:draft, :active, :archived]
    field :amount_cents, :integer  # Never :float for money!
    belongs_to :user, MyApp.Accounts.User
    timestamps(type: :utc_datetime_usec)
  end

  def changeset(entity, attrs) do
    entity
    |> cast(attrs, [:name, :status, :amount_cents])
    |> validate_required([:name])
    |> foreign_key_constraint(:user_id)
  end
end

© oliver-kriska, MIT. Rendered from Markdown: HTML in the file is shown as text, images as links, and headings moved down two levels. Raw file

Files

SKILL.md and 5 other files (references) in targets/pi/skills/ecto-patterns of oliver-kriska/claude-elixir-phoenix.

• SKILL.md
• references/changesets.md
• references/fulltext-search.md
• references/migrations.md
• references/queries.md
• references/transactions.md

Open the folder on GitHubat commit 9767a82