Build your own library to render JSON response in Phoenix

In my previous article, I introduced my library call JsonView to render json response easier.

Today I will guide you to write your own Json render view. Let's start.

Now for example I have a Blog app with User Category , Post and Comment schemas.

This is PostView which is generated by Phoenix

defmodule MyBlogWeb.PostView do
  use MyBlogWeb, :view
  alias MyBlogWeb.PostView

  def render("index.json", %{posts: posts}) do
    %{data: render_many(posts, PostView, "post.json")}
  end

  def render("show.json", %{post: post}) do
    %{data: render_one(post, PostView, "post.json")}
  end

  def render("post.json", %{post: post}) do
    %{id: post.id,
      title: post.title,
      description: post.description,
      content: post.content,
      cover: post.cover,
      is_published: post.is_published}
  end
end

Let's improve it

1. Use `Map.take`

...
def render("post.json", %{post: post}) do
        Map.take(post, [:id, :title, :description, :content, :cover, :is_published])
end
...

This way you don't have to write much code every time you add a new attribute.

2. Render custom field

You may want to:

Format some field value instead of return original value
Calculate virtual field

Normally you will do this:

def render("post.json", %{post: post}) do
    post
    |> Map.take([:id, :title, :description, :content, :cover, :is_published])
    |> Map.merge(%{
      comment_count: render_comment_count(post),
      author_name: render_author_name(post)
    })
  end

  def render_comment_count(post) do
    ...
  end

  def render_author_name(post) do
    ...
  end

Or you can reduce a bit of code by using pattern matching to render custom field value

def render("post.json", %{post: post}) do
    post
    |> Map.take([:id, :title, :description, :content, :cover, :is_published])
    |> Map.merge(render_custom_fields(post, [:comment_count, :author_name]))
  end

  defp render_custom_fields(struct, fields) do
    Enum.map(fields, fn field ->
        {field, render_field(field, struct)}
    end)
    |> Enum.into(%{})
  end

  defp render_field(:comment_count, post) do
    ...
  end

  defp render_field(:author_name, post) do
    ...
  end

Now every time you add a new custom field, just add field name to the list, and define a render_field/2 function

3. Render relation ship

You may want to return the whole object of author. For example you have a view UserView so you can do:

def render("post.json", %{post: post}) do
    post
    ...
    |> Map.merge(%{
      author: render_one(post.author, MyBlogWeb.UserView, "user.json")
    })
 end

It requires that author must be loaded, if not, you will get this error

** (KeyError) key :id not found in: #Ecto.Association.NotLoaded<association :author is not loaded>

You can handle it by pattern matching against Ecto.Association.NotLoaded

def render("post.json", %{post: post}) do
    post
    ...
    |> Map.merge(%{
      author: render_relationship(post.author, MyBlogWeb.UserView, "user.json")
    })
 end

 defp render_relationship(%Ecto.Association.NotLoaded{}, _, _), do: nil

 defp render_relationship(relation, view, template) do
    render_one(relation, view, template)
 end

And it only render relations struct if loaded, otherwise it is set to nil.

Now you can improve it to render list of relationships

def render("post.json", %{post: post}) do
    post
    ...
    |> Map.merge(
      render_relationship(post, [
        {:author, MyBlogWeb.UserView, "user.json"},
        {:comments, MyBlogWeb.CommentView, "comment.json"}
      ])
    )
end

defp render_relationship(struct, relationships) do
    Enum.map(relationships, fn {field, view, template} ->
      {field, render_relationship(Map.get(struct, field), view, template)}
    end)
    |> Enum.into(%{})
end

defp render_relationship(%Ecto.Association.NotLoaded{}, _, _), do: nil

defp render_relationship(relations, view, template) when is_list(relations) do
    render_many(relations, view, template)
end

defp render_relationship(relation, view, template) do
    render_one(relation, view, template)
end

With this way you can handle both single struct and list of struct.

4. Combines these helper functions

You can combine them all in one function and only need to pass field definition to this function

@fields [:id, :title, :description, :content, :cover, :is_published]
  @custom_fiels [:comment_count, :author_name]
  @relationships [
    {:author, MyBlogWeb.UserView, "user.json"},
    {:comments, MyBlogWeb.CommentView, "comment.json"}
  ]

  def render("post.json", %{post: post}) do
    render_json(post, @fields, @custom_fiels, @relationships)
  end

  def render_json(struct, fields, custom_fields \\ [], relationships \\ []) do
    struct
    |> Map.take(fields)
    |> Map.merge(render_custom_fields(struct, custom_fields))
    |> Map.merge(render_relationship(struct, relationships))
  end

Move them to a helper

These functions are the same for every view, so let's move these code to a helper module JsonViewHelper

defmodule JsonViewHelper do
  import Phoenix.View, only: [render_one: 3, render_many: 3]

  def render_json(struct, view, fields, custom_fields \\ [], relationships \\ []) do
    struct
    |> Map.take(fields)
    |> Map.merge(render_custom_fields(struct, view, custom_fields))
    |> Map.merge(render_relationship(struct, relationships))
  end

  defp render_custom_fields(struct, view, fields) do
    Enum.map(fields, fn field ->
      {field, view.render_field(field, struct)}
    end)
    |> Enum.into(%{})
  end

  defp render_relationship(struct, relationships) do
    Enum.map(relationships, fn {field, view, template} ->
      {field, render_relationship(Map.get(struct, field), view, template)}
    end)
    |> Enum.into(%{})
  end

  defp render_relationship(%Ecto.Association.NotLoaded{}, _, _), do: nil

  defp render_relationship(relations, view, template) when is_list(relations) do
    render_many(relations, view, template)
  end

  defp render_relationship(relation, view, template) do
    render_one(relation, view, template)
  end
end

Here I modify render_custom_fields a bit, because we call render_field to render custom field, so we have pass the view module as second parameter, so we can use the module to invoke those render_field that we define.

And now render json response is much simple:

defmodule BlogeeWeb.PostView do
    ...
  @fields [:id, :title, :description, :content, :cover]
  @custom_fields [:status]
  @relationships [
    {:author, BlogeeWeb.UserView, "basic_info.json"},
    {:category, BlogeeWeb.CategoryView, "category.json"}
  ]
  def render("post.json", %{post: post}) do
    JsonViewHelper.render_json(post, __MODULE__, @fields, @custom_fields, @relationships)
  end

  def render_field(:status, post) do
    if post.is_published do
      "published"
    else
      "draft"
    end
  end
end