ecto-changesets

# Ecto Changesets

Master Ecto changesets to validate, cast, and transform data before database operations.
This skill covers changeset creation, validation, constraints, handling associations,
and advanced patterns for maintaining data integrity.

## Basic Changeset

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

  schema "users" do
    field :name, :string
    field :email, :string
    field :age, :integer

    timestamps()
  end

  def changeset(user, params \\ %{}) do
    user
    |> cast(params, [:name, :email, :age])
    |> validate_required([:name, :email])
  end
end

# Usage
changeset = MyApp.User.changeset(%MyApp.User{}, %{name: "John", email: "john@example.com"})
```

Changesets filter and validate parameters before they're applied to a struct.
The `cast/3` function specifies which fields can be changed, and `validate_required/2`
ensures specific fields are present.

## Creating and Validating Changesets

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

  schema "people" do
    field :first_name, :string
    field :last_name, :string
    field :age, :integer

    timestamps()
  end

  def changeset(person, params \\ %{}) do
    person
    |> cast(params, [:first_name, :last_name, :age])
    |> validate_required([:first_name, :last_name])
    |> validate_number(:age, greater_than_or_equal_to: 0)
  end
end

# Create changeset
changeset = MyApp.Person.changeset(%MyApp.Person{}, %{first_name: "Jane"})

# Check validity
changeset.valid?  # false, last_name is missing

# Access errors
changeset.errors
# [first_name: {"can't be blank", [validation: :required]},
#  last_name: {"can't be blank", [validation: :required]}]
```

The `valid?` field indicates whether the changeset has any errors. The `errors`
field contains a keyword list of validation failures with error messages and metadata.

## Inserting with Changesets

```elixir
person = %MyApp.Person{}
changeset = MyApp.Person.changeset(person, %{
  firs