Elixir EXTO变更集和多对多关联“无效”错误

我有多对多关联的银行和货币模式。我用select为银行创建了一个包含货币的表单

bank.ex

schema "banks" do
  field :name, :string
  many_to_many :currencies, App.Currency, join_through: "banks_currencies"
end

@required_fields ~w(name)
@optional_fields ~w()

def changeset(model, params \\ %{}) do
  model
  |> cast(params, @required_fields, @optional_fields)
  |> cast_assoc(:currencies)
end

def changeset(model, params \\ %{}) do

  currencies = Currency
  |> Currency.by_ids(if params["currencies"], do: params["currencies"], else: [])
  |> App.Repo.all
  |> Enum.map(&Ecto.Changeset.change/1)

  model
  |> cast(params, @required_fields, @optional_fields)
  |> put_assoc(:currencies, currencies)
end

form.html.ex

<%= multiple_select(form, :currencies, @currencies) %>

问题是：当我提交选择了某些货币的表单时，变更集返回的是无效错误。我在iex中模拟：

iex(8)> App.Bank.changeset bank, %{"currencies" => ["1"]}  
#Ecto.Changeset<action: nil, changes: %{},
 errors: [currencies: {"is invalid", [type: {:array, :map}]}],
 data: #App.Bank<>, valid?: false>

---

我安装了最新的Phoenix，最近从1.1.4升级到1.2。我不确定它是不是正确的解决方案，但它可以工作

bank.ex

schema "banks" do
  field :name, :string
  many_to_many :currencies, App.Currency, join_through: "banks_currencies"
end

@required_fields ~w(name)
@optional_fields ~w()

def changeset(model, params \\ %{}) do
  model
  |> cast(params, @required_fields, @optional_fields)
  |> cast_assoc(:currencies)
end

def changeset(model, params \\ %{}) do

  currencies = Currency
  |> Currency.by_ids(if params["currencies"], do: params["currencies"], else: [])
  |> App.Repo.all
  |> Enum.map(&Ecto.Changeset.change/1)

  model
  |> cast(params, @required_fields, @optional_fields)
  |> put_assoc(:currencies, currencies)
end

货币兑换率

def by_ids(query, ids) do
  from c in query, where: c.id in ^ids
end