Poison.Encoder如何預加載關聯？

Question

我有下面的Ecto模型。當我嘗試渲染時出現錯誤。我如何修改@derive以便預加載？或者我必須寫出實施？推薦的處理方法是什麼？

** (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 

該模型是在這裏：

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

簡短的回答是，你不應該。預加載數據不是視圖層的責任。

當你提取你的資源使用Ecto.Repo.preload/2您應該執行預緊

例如：

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

您也可以進行預加載（通常是控制器或從控制器調用的函數。）在Ecto.Query.preload/3查詢：

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