Poison.Encoder如何预加载关联？

Question

下面是经济学模型。当我试图渲染时，我会得到一个错误。我如何修改“派生”以使其能够预加载？还是我必须写出实现？建议的处理方法是什么？

** (RuntimeError) cannot encode association :tilemap_layers from MyProject.Tilemap to JSON because the association was not loaded. Please make sure you have preloaded the association or remove it from the data to be encoded

模型如下：

defmodule MyProject.Tilemap do
  use MyProject.Web, :model

  @derive {Poison.Encoder, only: [
    :name,
    :tile_width,
    :tile_height,
    :width,
    :height,
    :orientation,
    :tilemap_layers,
    :tilesets
  ]}

  schema "tilemaps" do

    field :name, :string
    field :tile_width, :integer
    field :tile_height, :integer
    field :width, :integer
    field :height, :integer
    field :orientation, :string

    has_many :tilemap_layers, MyProject.TilemapLayer
    has_many :tilesets, MyProject.Tileset

    timestamps
  end

  @required_fields ~w(tile_width tile_height width height)
  @optional_fields ~w()

  @doc """
  Creates a changeset based on the `model` and `params`.

  If no params are provided, an invalid changeset is returned
  with no validation performed.
  """
  def changeset(model, params \\ :empty) do
    model
    |> cast(params, @required_fields, @optional_fields)
  end
end

Answer 1

简单地说，你不应该。预装数据不是视图层的责任。

当您获取资源时(通常是控制器或从控制器调用的函数)，您应该执行预加载。

例如，使用预加载/3

def index(_conn, _params)
  timemaps = Tilemap |> Repo.all() |> Repo.preload(:timemap_layers)
  render("index.json", tilemaps: tilemaps)
end

还可以在使用预加载/3的查询中执行预加载。

query = from t in Tilemap,
  preload: [:tilemap_layers]
Repo.all(query)