Ecto Patterns

---
name: ecto-patterns
description: "Ecto patterns — schemas, changesets, queries, migrations, Multi, associations, preloads, upserts. Use when editing Repo calls, Ecto.Query, or schema fields. 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… install to load the full skill