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

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