Conditionally construct map

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1

So I have found a pothole and I am not sure what the solution is.

I have a pretty standard JSON api in phoenix, I am using ecto, and I am mapping those to maps inside the ThingJSON this is all pretty standard.

Now depending on the route, the controller might decide it doesn’t want to preload some data. For example

%Aos.Schema.Shipyard{
  __meta__: #Ecto.Schema.Metadata<:loaded, "shipyard">,
  id: "103b0451-7396-4f11-9cf8-7081c1829f5e",
  port_id: "a748b9d2-40ff-4f14-8c8c-258b343145eb",
  port: %Aos.Schema.Port{
    __meta__: #Ecto.Schema.Metadata<:loaded, "port">,
    id: "a748b9d2-40ff-4f14-8c8c-258b343145eb",
    name: "London",
    shortcode: "lond",
    destinations: #Ecto.Association.NotLoaded<association :destinations is not loaded>,
    ships: #Ecto.Association.NotLoaded<association :ships is not loaded>,
    shipyard: #Ecto.Association.NotLoaded<association :shipyard is not loaded>,
    agents: #Ecto.Association.NotLoaded<association :agents is not loaded>,
    inserted_at: ~U[2024-07-01 02:02:57Z],
    updated_at: ~U[2024-07-01 02:02:57Z]
  },
  ships: [
    %Aos.Schema.ShipyardStock{
      __meta__: #Ecto.Schema.Metadata<:loaded, "shipyard_stock">,
      id: "8117b318-b0ef-4aa7-be31-88437acbe8fc",
      shipyard_id: "103b0451-7396-4f11-9cf8-7081c1829f5e",
      shipyard: #Ecto.Association.NotLoaded<association :shipyard is not loaded>,
      ship_id: "0a238468-1cf0-47b4-b891-d5936d6554a8",
      ship: #Ecto.Association.NotLoaded<association :ship is not loaded>,
      cost: 100,
      inserted_at: ~U[2024-07-01 02:02:57Z],
      updated_at: ~U[2024-07-01 02:02:57Z]
    }
  ],
  inserted_at: ~U[2024-07-01 02:02:57Z],
  updated_at: ~U[2024-07-01 02:02:57Z]
}

On one route it might want to preload the :ships but on another it might not want to.

I have some pretty standard view code that looks like this


  def render("shipyards.json", %{page: page}) do
    %{
      data: for(yard <- page.entries, do: shipyard(yard)),
      meta: %{
        page_number: page.page_number,
        page_size: page.page_size,
        total_entries: page.total_entries,
        total_pages: page.total_pages
      }
    }
  end

  def shipyard(shipyard) do
    %{
      id: shipyard.id,
      port: PortJSON.port(shipyard.port)
    }
  end

Now I want to modify the shipyard function to also display the ships but only if the assoc is actually preloaded.

Okay so I think

  def shipyard(shipyard) do
    %{
      id: shipyard.id,
      port: PortJSON.port(shipyard.port),
      ships:
        if Ecto.assoc_loaded?(shipyard.ships) do
          for(entry <- shipyard.ships, do: stock(entry))
        else
          # ???
        end
    }
  end

Problem is, if ships is not loaded it will export json of ships: null which is misleading IMO. I’d rather it just omitted the key.

I could do something like this

  def render_if(map, key, assoc, render) do
    if Ecto.assoc_loaded?(assoc) do
      Map.put(map, key, render.(assoc))
    else
      map
    end
  end

  def shipyard(shipyard) do
    %{
      id: shipyard.id,
      port: PortJSON.port(shipyard.port)
    }
    |> render_if(:ships, shipyard.ships, &stock/1)
  end

But it feels kinda wrong.

Also why is Ecto.assoc_loaded? not available as a guard.

Anyone got any ideas?

2

I don’t think there’s anything wrong with the way you’re handling it. I have done similar although I had a util that constructed the json with variable guards, among them filtering out unloaded associations–something like this:

def serialize(data) do
  Map.flat_map(data, fn 
    {field, %Ecto.AssocNotLoaded{}} -> []
    {field, assoc = %{}} -> [{field, serialize(assoc)}]
    {field, val} -> [{field, val}]
  end)
  # other selecting/filtering...
end
3 Likes
3

You can pattern match on Ecto.AssocNotLoaded and then have the map construction not include the key in that clause, or you can put nil and then have Enum.reject that deletes any map pairs with a nil value. Either one would be just fine.

3 Likes
4

I’d likely use a filter or guard is_list(shipyard.ships)