Ecto Patterns

What the source does

• FilesystemNone observedNo reads or writes in the source
• NetworkNone observedNo outbound calls in the source
• ShellNone observedNo commands run in the source
• DependenciesPinnedNothing floats
• SecretsNone foundNo keys in source

Five audit layers

Gates applied: no behavioural pass.

Audit 0.4.1 · source sha 492f17636999 · 18 Oct 2026Report an issue or request a re-scan

• Publisheroliver-kriska GitHub userOwner of the repository this skill is read from
• Source folderoliver-kriska/claude-elixir-phoenix/plugins/elixir-phoenix/skills/ecto-patternsThe exact folder we audited
• LicenceMITAs the repository states it
• Commit pinned for this verdict492f17636999A new audit runs when the source changes
• Repository statusNot archivedLast push 2 days ago
• Sandboxed runNot runSkills are read, not executed; layer L4 is skipped

Its 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.

---
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."
effort: medium
user-invocable: false
paths:
  - "**/migrations/*.exs"
  - "**/*_schema.ex"
  - "**/*changeset*.ex"
---

# Ecto Patterns Reference

Reference for working with Ecto schemas, queries, and migrations.

## Iron Laws — Never Violate These

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

## Quick Schema Template

```elixir
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
```

## Quick Decisions

### cast vs put_change vs change

| Function | Use When |
|----------|----------|
| `cast/4` | External data (user input, API) |
| `put_change/3` | Internal trusted data (timestamps, computed) |
| `change/2` | Internal data from existing struct |

### Preload Strategy

| Relationship | Strategy |
|--------------|----------|
| `belongs_to` | JOIN (single query) |
| `has_many` | Separate queries (avoid row multiplication) |

## Common Anti-patterns

| Wrong | Right |
|-------|-------|
| `field :amount, :float` | `field :amount_cents, :integer` |
| `"SELECT * WHERE name = '#{name}'"` | `from(u in User, where: u.name == ^name)` |
| `Repo.all(User) \|> Enum.filter(& &1.active)` | `from(u in User, where: u.active)` |
| Preloading in loops | `Repo.preload(posts, :comments)` |
| `Repo.get!(User, user_id)` with user input | `Repo.get(User, id)` + handle nil |
| `{:ok, _} = Repo.update(cs)` inside `Repo.transaction` | `case`/`with` + `Repo.rollback(cs)` (see transactions.md) |

## References

For detailed patterns, see:

- `${CLAUDE_SKILL_DIR}/references/changesets.md` - cast vs put_change, custom validations, prepare_changes
- `${CLAUDE_SKILL_DIR}/references/queries.md` - Composable queries, dynamic, subqueries, preloading
- `${CLAUDE_SKILL_DIR}/references/migrations.md` - Safe migrations, concurrent indexes, NOT NULL
- `${CLAUDE_SKILL_DIR}/references/transactions.md` - Repo.transact, Ecto.Multi, upserts

Excerpt, first 3,624 characters of SKILL.md, verbatim from the audited commit. It is text from someone else’s repository, quoted so you can judge it before your agent reads it.

git clone https://github.com/oliver-kriska/claude-elixir-phoenix
git -C claude-elixir-phoenix checkout 492f17636999

Works in

1. 2026-10-18
   ✓SAFE B 89 / 100

   first audit · source 492f17636999

Questions about Ecto Patterns

• github.com/oliver-kriska/claude-elixir-phoenix/tree/main/plugins — the audited source at commit 492f17636999 and every finding aboveObserved · 18 Oct 2026

Spotted something wrong? Report a correction · Methodology