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