How to not encode nil field from Struct using Jason

Questions / Help
, ,
1

Background

I have a TypedStruct with an optional field, :rank. This field is sometimes present and sometimes not.

  @derive Jason.Encoder
  typedstruct enforce: true do
    @typedoc "An order."

    field(:item_id, String.t())
    field(:rank, non_neg_integer(), enforce: false)
  end

The ideal situation here would be to generate the struct without the field, should the :rank parameter be missing. However, since I am dealing with structs, this is not possible, so I have to contempt with the nil value representing the absence of :rank.

The problem here is that when I use Jason.encode(...) , I am going to get a "rank": null in the result json object. I really don’t want this.

Question

Is there an option in Jason that allows me to not encode nil values, other than manually implementing the Encode Protocol myself?

2

other than manually implementing the Encode Protocol myself

From the “Deriving” section of the Jason.Encoder docs, there doesn’t appear to be a way.

However, Jason.Encode.map/2 appears to make it incredibly simple to implement Jason.Encoder on your own maps. It seems like something like this would do the trick:

defimpl Jason.Encoder do
  def encode(value, opts) do
    value
    |> Map.from_struct() # or take exactly the keys you want
    |> Map.reject(fn {_k, v} -> is_nil(v) end) # since 1.13
    |> Jason.Encode.map(opts)
  end
end
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