在Phoenix Framework中呈现多对多关系JSON

问题描述

我有下一个型号

defmodule App.User do
  use App.Web, :model
  alias App.User

  schema "users" do  
    field :name, :string
    has_many :roles_users, App.RolesUser
    has_many :roles, through: [:roles_users, :role]
    timestamps
  end
end 

defmodule App.Role do
  use App.Web, :model

  schema "roles" do
    has_many :roles_users, App.RolesUser
    has_many :users, through: [:roles_users, :user]
    field :name, :string
    timestamps
  end

end

defmodule App.RolesUser do
  use App.Web, :model

  schema "roles_users" do
    belongs_to :role, App.Role
    belongs_to :user, App.User
    timestamps
  end
end

是多对多关系.我要显示的控制器是

Is for a many to many relationship. My controller to show is

def index(conn, _params) do
  users = Repo.all(User)
        |> Repo.preload(:roles)

  render(conn, "index.json", users: users)
end

我认为

def render("index.json", %{users: users}) do
  %{users: render_many(users, App.UserView, "user.json")}
end

def render("show.json", %{user: user}) do
  %{user: render_one(user, App.UserView, "user.json")}
end

def render("user.json", %{user: user}) do
  %{id: user.id,
    name: user.name,
    roles: user.roles
 }

当我发送GET请求时出现此错误

When I sent the GET request I got this error

unable to encode value: {nil, "roles"}

我知道这可能是因为user.roles需要以某种方式格式化才能解码JSON，但是我对此一无所知.我尝试过这种形式

I know that could be because user.roles need to be formatted in some way to decode the JSON, but I have not any clue about this. I have tried in the form

def render("user.json", %{user: user}) do
  %{id: user.id,
    name: user.name,
    roles: render_many(roles, App.UserView, "roles.json")
 }

但是不起作用.

在视图中呈现多对多关系的最佳方法是什么?

What is the best way to render many to many relationships in view?

推荐答案

使用 render_many/4 是正确的.

如果您希望在同一模块中定义"role.json"渲染函数，则可以执行以下操作:

If you wish to define the "role.json" render function in the same module you can do:

def render("user.json", %{user: user}) do
  %{
    id: user.id,
    name: user.name,
    roles: render_many(user.roles, __MODULE__, "role.json", as: :role)
  }
end

def render("role.json", %{role: role}) do
  %{
    id: role.id
    ... 
  }
end

请注意，我们将as: :role传递给render_many函数.这是因为从视图名称推断出assign(%{role: role})部分.在这种情况下，它是UserView，因此默认情况下为%{user: user}.

Notice that we pass as: :role to the render_many function. This is because the assigns (the %{role: role}) part is inferred from the view name. In this case it is the UserView so it would be %{user: user} by default.

如果定义了RoleView模块，则只需将def render("role.json")函数移至新的RoleView并调用render_many而不使用as选项:

If you define a RoleView module then you can just move the def render("role.json") function to your new RoleView and call render_many without the as option:

...
roles: render_many(user.roles, MyApp.RoleView, "role.json")
...

您可能更喜欢的另一种选择是在模型中导出协议:

Another option that may be preferable for you is to derive a protocol in your model:

defmodule App.Role do
  use App.Web, :model
  @derive {Poison.Encoder, only: [:id, :name]}

  schema "roles" do
    has_many :roles_users, App.RolesUser
    has_many :users, through: [:roles_users, :user]
    field :name, :string
    timestamps
  end

我个人认为这会将您的模型与您的视图结合起来，所以我更喜欢使用第一个选项.

Personally I feel this couples your model to your view, so I prefer to use the first option.

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