{"slug":"ecto-patterns-2","title":"ecto-patterns","summary":"'Use when a task touches Ecto, even a how-to question: schemas, changesets,","platform":"Claude","tags":[],"authorName":"LLM Mart","authorSlug":"llm-mart","score":0,"source":"github","price":null,"verified":false,"createdAt":"2026-10-04T15:14:21.820613Z","repo":{"url":"https://github.com/oliver-kriska/claude-elixir-phoenix","stars":560,"forks":44,"license":"MIT","updatedAt":"2026-10-02T04:11:38Z"},"bodyHtml":"
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name: ecto-patterns\ndescription: 'Use when a task touches Ecto, even a how-to question: schemas, changesets,\nvalidations, queries, preloads, Multi, migrations, constraints, money fields. Load\nit before writing Ecto code. Skip for Ash.'

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Ecto Patterns Reference

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Reference for working with Ecto schemas, queries, and migrations.

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Iron Laws — Never Violate These

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

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defmodule MyApp.Context.Entity do\n  use Ecto.Schema\n  import Ecto.Changeset\n\n  @primary_key {:id, :binary_id, autogenerate: true}\n  @foreign_key_type :binary_id\n\n  schema \"entities\" do\n    field :name, :string\n    field :status, Ecto.Enum, values: [:draft, :active, :archived]\n    field :amount_cents, :integer  # Never :float for money!\n    belongs_to :user, MyApp.Accounts.User\n    timestamps(type: :utc_datetime_usec)\n  end\n\n  def changeset(entity, attrs) do\n    entity\n    |> cast(attrs, [:name, :status, :amount_cents])\n    |> validate_required([:name])\n    |> foreign_key_constraint(:user_id)\n  end\nend\n
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Quick Decisions

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cast vs put_change vs change

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FunctionUse When
cast/4External data (user input, API)
put_change/3Internal trusted data (timestamps, computed)
change/2Internal data from existing struct
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Preload Strategy

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RelationshipStrategy
belongs_toJOIN (single query)
has_manySeparate queries (avoid row multiplication)
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Common Anti-patterns

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WrongRight
field :amount, :floatfield :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 loopsRepo.preload(posts, :comments)
Repo.get!(User, user_id) with user inputRepo.get(User, id) + handle nil
{:ok, _} = Repo.update(cs) inside Repo.transactioncase/with + Repo.rollback(cs) (see transactions.md)
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References

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For detailed patterns, see:

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