creating json from elixir ecto associations

Question

I would like to generate JSON from an ecto association in phoenix.

this is my association:

defmodule Blog.Post do
  use Ecto.Model

  schema "posts" do
    field :title, :string
    field :body, :string
    has_many :comments, Blog.Comment
  end
end

and:

defmodule Blog.Comment do
  use Ecto.Model

  schema "comments" do
    field :content, :string
    belongs_to :post, Blog.Post
  end
end

when I generate json without the association the result is this:

[%Blog.Post{body: "this is the very first post ever!", id: 1,title: "first post"},
 %Blog.Post{body: "Hello nimrod!!!!", id: 12, title: "hi Nimrod"},
 %Blog.Post{body: "editing the body!!!!", id: 6, title: "hello(edit)"}]

and the json looks like this

{"posts": [
    {
        "title": "first post",
        "id": 1,
        "body": "this is the very first post ever!"
    },
    {
        "title": "hi Nimrod",
        "id": 12,
        "body": "Hello nimrod!!!!"
    },
    {
        "title": "hello(edit)",
        "id": 6,
        "body": "editing the body!!!!"
    }
]}

but with the association the outcome is this

[%Blog.Post{body: "this is the very first post ever!",
 comments: {Ecto.Associations.HasMany.Proxy,
 #Ecto.Associations.HasMany<[name: :comments, target: Blog.Post,
 associated: Blog.Comment, references: :id, foreign_key: :post_id]>}, id: 1,
 title: "first post"},
 %Blog.Post{body: "Hello nimrod!!!!",
 comments: {Ecto.Associations.HasMany.Proxy,
 #Ecto.Associations.HasMany<[name: :comments, target: Blog.Post,
 associated: Blog.Comment, references: :id, foreign_key: :post_id]>}, id: 12,
 title: "hi Nimrod"},
 %Blog.Post{body: "editing the body!!!!",
 comments: {Ecto.Associations.HasMany.Proxy,
 #Ecto.Associations.HasMany<[name: :comments, target: Blog.Post,
 associated: Blog.Comment, references: :id, foreign_key: :post_id]>}, id: 6,
 title: "hello(edit)"}]

With the above output I can't create a proper json output. I would like for the json to look something like this

{"posts": [
     {
        "title": "the title",
        "id": 1,
        "body": "the body",
        "comments": [{"content": "a comment"}, {"content": "another comment"}]
     }
     ...
]}

Any help would be appreciated.

Answer 1

Ouch, there is no easy solution at this moment. I would try something like:

defimpl Poison.Encoder, for: Tuple do
  def encode(proxy, options) do
    Poison.Encoder.List.to_json(proxy.all, options)
  end
end

We are basically implementing encoder for tuples that receives the proxy above and encoding all items. We need to discuss better solutions for this.