Ecto Patterns Skill for Claude

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:

• references/changesets.md - cast vs put_change, custom validations, prepare_changes
• references/queries.md - Composable queries, dynamic, subqueries, preloading
• references/migrations.md - Safe migrations, concurrent indexes, NOT NULL
• references/transactions.md - Repo.transact, Ecto.Multi, upserts