Any alternatives for @derive and Poison.Encoder?

Questions / Help
,
1

Hi guys!

In this moment I am working in a large umbrella application composed for around 4 applications each one has their own models that are using this implementation for Poison.Encoder

defimpl Poison.Encoder, for: Any do
    def encode(%{__struct__: App.Models.Task} = struct, options) do
    struct =
        if struct.geom do
            {lon, lat} =  struct.geom.coordinates
            %{struct | lon: lon, lat: lat}
        else
            struct
        end
        encode_model(struct, options)
    end

    def encode(%{__struct__: _} = struct, options) do
        encode_model(struct, options)
    end

    def encode({lon, lat}, options) when is_float(lon) and is_float(lat) do
        %{lon: lon, lat: lat}
        |> sanitize_map()
        |> Poison.Encoder.Map.encode(options)
    end

    defp encode_model(model, options) do
        model
        |> Map.from_struct()
        |> sanitize_map()
        |> Poison.Encoder.Map.encode(options)
    end

    defp sanitize_map(map) do
        map
        |> Enum.filter(
            fn({_, %Ecto.Association.NotLoaded{}}) -> false
            ({:__meta__, _}) -> false
            ({:__struct__, _}) -> false
            ({_, _}) -> true
           end)
    |> Map.new()
    end
end

My model looks like:

# App.Models.Task
@derive {Poison.Encoder, only: [
  :address,
  :address_extra,
  :geom,
  :country,
  :dep_state,
  :city,
  :zip_code,
  :lat,
  :lon
]}
schema "tasks" do
  field(:address, :string)
  field(:address_extra, :string)
  field(:geom, Geo.Geometry)
  field(:country, :string)
  field(:dep_state, :string)
  field(:city, :string)
  field(:zip_code, :string)
  field(:lat, :float, virtual: true)
  field(:lon, :float, virtual: true)
  
  belongs_to(:service, App.Models.Service)
  timestamps()
end

This obviously doesn’t work once I want to compile my releases (using distillery) since I’m overriding the existing encoder implementations. So, in order to fix the issue I will need to implement Poison.Encoder using @derive in my models.

So the thing is, I know preloading the relationships is the way to go, but, what if I don’t want to preload all the relationships of my models? There are another way to go? If this is the only way, at least can I skip preloading relationships?

Thank you so much in advance for your time.

2

In general the protocol implementation is a bad idea for encoding domain entities like ecto structs - it’s very likely you’d like to differ how you encode them in different situations and protocols are global.

The way to go is to use phoenix views to translate from structs to simple data that can be encoded - take a look at the default code phoenix generates when you use phx.gen.json.

3 Likes
3

Hi and thanks for the quick response @michalmuskala.