elixir-ecto-patterns

# Elixir Ecto Patterns

Master Ecto, Elixir's database wrapper and query generator. This skill
covers schemas, changesets, queries, associations, and transactions for
building robust database applications.

## Schema Definition

```elixir
defmodule MyApp.User do
  use Ecto.Schema
  import Ecto.Changeset

  schema "users" do
    field :name, :string
    field :email, :string
    field :age, :integer
    field :is_active, :boolean, default: true
    field :role, Ecto.Enum, values: [:user, :admin, :moderator]

    has_many :posts, MyApp.Post
    belongs_to :organization, MyApp.Organization

    timestamps()
  end

  def changeset(user, attrs) do
    user
    |> cast(attrs, [:name, :email, :age, :is_active, :role])
    |> validate_required([:name, :email])
    |> validate_format(:email, ~r/@/)
    |> validate_number(:age, greater_than: 0, less_than: 150)
    |> unique_constraint(:email)
  end
end
```

## Changeset Validations

```elixir
defmodule MyApp.Post do
  use Ecto.Schema
  import Ecto.Changeset

  schema "posts" do
    field :title, :string
    field :body, :text
    field :published, :boolean, default: false
    field :tags, {:array, :string}, default: []

    belongs_to :user, MyApp.User

    timestamps()
  end

  def changeset(post, attrs) do
    post
    |> cast(attrs, [:title, :body, :published, :tags, :user_id])
    |> validate_required([:title, :body, :user_id])
    |> validate_length(:title, min: 3, max: 100)
    |> validate_length(:body, min: 10)
    |> foreign_key_constraint(:user_id)
  end

  def publish_changeset(post) do
    post
    |> change(published: true)
  end
end
```

## Basic Queries

```elixir
import Ecto.Query

# Get all users
Repo.all(User)

# Get user by ID
Repo.get(User, 1)
Repo.get!(User, 1)  # Raises if not found

# Get by specific field
Repo.get_by(User, email: "user@example.com")

# Filter with where clause
query = from u in User, where: u.age > 18
Repo.all(query)

# Select specific fields
query = from u in User, select: {u.id, u.nam