ecto-patterns

# 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 :...