Specs

cast(
  Ecto.Schema.t() | t() | {data(), types()},
  %{required(binary()) => term()} | %{required(atom()) => term()} | :invalid,
  [atom()],
  Keyword.t()
) :: t()

Applies the given params as changes for the given data according to the given set of permitted keys. Returns a changeset.

The given data may be either a changeset, a schema struct or a {data, types} tuple. The second argument is a map of params that are cast according to the type information from data. params is a map with string keys or a map with atom keys containing potentially invalid data.

During casting, all permitted parameters whose values match the specified type information will have their key name converted to an atom and stored together with the value as a change in the :changes field of the changeset. All parameters that are not explicitly permitted are ignored.

If casting of all fields is successful, the changeset is returned as valid.

Note that cast/4 validates the types in the params, but not in the given data.

Options

• :empty_values - a list of values to be considered as empty when casting. Empty values are always replaced by the default value of the respective key. Defaults to [""]

Examples

iex> changeset = cast(post, params, [:title])
iex> if changeset.valid? do
...>   Repo.update!(changeset)
...> end

Passing a changeset as the first argument:

iex> changeset = cast(post, %{title: "Hello"}, [:title])
iex> new_changeset = cast(changeset, %{title: "Foo", body: "World"}, [:body])
iex> new_changeset.params
%{"title" => "Hello", "body" => "World"}

Or creating a changeset from a simple map with types:

iex> data = %{title: "hello"}
iex> types = %{title: :string}
iex> changeset = cast({data, types}, %{title: "world"}, [:title])
iex> apply_changes(changeset)
%{title: "world"}

Composing casts

cast/4 also accepts a changeset as its first argument. In such cases, all the effects caused by the call to cast/4 (additional errors and changes) are simply added to the ones already present in the argument changeset. Parameters are merged (not deep-merged) and the ones passed to cast/4 take precedence over the ones already in the changeset.