小编Roe*_*mer的帖子

如何与ecto 2建立多对多关系？作为一个示例应用程序,我想创建一个可以在多个类别中的帖子.这些类别已经存在.例如:

[%Category{id: "1", name: "elixir"}, %Category{id: "2", name: "erlang"}]

我正在使用Ecto 2 beta 0.示例项目名为Ecto2.

我定义了两个模型:

defmodule Ecto2.Post do
  use Ecto2.Web, :model
  use Ecto.Schema

  schema "posts" do
    field :title, :string
    many_to_many :categories, Ecto2.Category, join_through: "posts_categories", on_replace: :delete
    timestamps
  end

  @required_fields ~w(title)
  @optional_fields ~w()
  def changeset(model, params \\ :empty) do
    model
    |> cast(params, @required_fields, @optional_fields)
    |> cast_assoc(:categories)  # not suitable?
  end
end

defmodule Ecto2.Category do
  use Ecto2.Web, :model

  schema "categories" do
    field :name, :string

    timestamps
  end

  @required_fields ~w(name)
  @optional_fields ~w()
  def changeset(model, …