Normally, you query data from database then render to JSON and return to client, and you might want to:

• Keep the original value
• Return value in a new format, or return some calculated data
• Render relationships that defined by Ecto schema

JsonView helps to render json data easier by support relationship and custom render data. Most of the time you may want to add it to your view:

def view do
  quote do
    ...
    use JsonView
    ...
  end
end

Or you can use it directly on your view

defmodule MyApp.PostView do
    use JsonView

    # define which fields return without modifying
    @fields [:title, :content, :excerpt, :cover]
    # define which fields that need to format or calculate, you have to define `render_field/2` below
    @custom_fields [:like_count]
    # define which view used to render relationship
    @relationships [author: MyApp.AuthorView]

    def render("post.json", %{post: post}) do
        # 1st way if `use JsonView`
        render_json(post, @fields, @custom_fields, @relationships)
    end

    def render_field(:like_count, item) do
        # load like_count from some where
    end
end

How to define fields and relationships

• Custom field

  @custom_fields [:like_count]
  # this invokes `render(:like_count, post)`
  @custom_fields [like_count: &my_function/1]
  # this invokes `my_function.(post)`

• Relationship

  @relationships [author: MyApp.UserView]
  # this invokes `MyApp.UserView.render("user.json", %{user: user})`
  @relationships [author: {MyApp.UserView, "basic_profile.json"}]
  # this invokes `MyApp.UserView.render("basic_profile.json", %{user: user})`

Data override JsonView render fields -> custom_fields -> relationships. If they define same field, then the latter will override the prior

Default fields You can pass a list of default fields and/or custom_fields as options to use JsonView. These fields then merged to fields and custom_fields before rendering data each time you invoke render_json

use JsonView, fields: [:id, :updated_at], custom_fields: [inserted_at: &to_local_time/2]

Render hook You can pass a function to process data after JsonView completes rendering like this:

use JsonView, after_render: &convert_all_datetime_to_local/1

def convert_all_datetime_to_local(data) do
  Enum.map(data, fn {k, v} ->
    v =
      case v do
        %NaiveDateTime{} -> to_local_datetime(v)
        _ -> v
      end
    {k, v}
  end)
  |> Enum.into(%{})
end