Ecto Patterns Reference
Reference for working with Ecto schemas, queries, and migrations.
Iron Laws — Never Violate These
- CHANGESETS ARE FOR EXTERNAL DATA — Use
cast/4 for user/API input, change/2 or put_change/3 for internal trusted data
- NEVER USE
:float FOR MONEY — Always use :decimal or :integer (cents)
- NO RAILS-STYLE POLYMORPHIC ASSOCIATIONS — They break foreign key constraints; use multiple nullable FKs or separate join tables
- ALWAYS PIN VALUES IN QUERIES —
u.name == ^user_input is safe, string interpolation causes SQL injection
- PRELOAD COLLECTIONS, NOT INDIVIDUALS — Preloading in loops = N+1 queries
- CONSTRAINTS BEAT VALIDATIONS FOR RACE CONDITIONS — Validations provide quick feedback, constraints provide DB-level safety
- SEPARATE QUERIES FOR
has_many, JOIN FOR belongs_to — Avoids row multiplication
- NO IMPLICIT CROSS JOINS —
from(a in A, b in B) without on: creates Cartesian product
- 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
