Decoding Json with nested lists of dissimilar structs

Questions / Help
, ,
1

Is there a built-in way to decode json that has a list of objects that are not decoded to the same struct, and the list is not at the root of the json?
Its a large, deeply nested json payload from a 3rd party, and I would prefer not to recursively decode and replace each list, but thats the only solution I have so far. Any suggestions would be appreciated.

Poison can do this if the list is the root,

defmodule TypeA do
  defstruct shared: "", only_a: ""
end

defmodule TypeB do
  defstruct shared: "", only_b: ""
end

options = fn
  %{only_a: _a} -> %TypeA{}
  %{only_b: _b} -> %TypeB{}
end

"""
[
  {
    "shared": "value",
    "only_a": "A!"
  }, 
  {
    "shared": "value",
    "only_b": "B!"
  }
]
"""
|> Poison.decode!([keys: :atoms!, as: [options]])

[
  %TypeA{shared: "value", only_a: "A!"}, 
  %TypeB{shared: "value", only_b: "B!"}
]

but if there is a parent struct, the children objects just decode to plain maps.

defmodule Parent do
  defstruct children: []
end

"""
{ 
  "children": [
    {
      "shared": "value",
      "only_a": "A!"
    }, 
    {
      "shared": "value",
      "only_b": "B!"
    }
  ]
}
"""
|> Poison.decode!( as: %Parent{})

%Parent{
  children: [
    %{"only_a" => "A!", "shared" => "value"}, 
    %{"only_b" => "B!", "shared" => "value"}
  ]
}

I’ve tried adding the as: function to the %Parent{} struct definition, but its not a valid value.

defmodule Parent do
  children = fn
    %{only_a: _a} -> %TypeA{}
    %{only_b: _b} -> %TypeB{}
  end

  defstruct children: children # <- invalid value
end
2

Another option would be a custom Poison’s encoder implementation for structs like Parent.

I would nevertheless stick with straight parsing into maps and then upgrading them to structs with update_in/3 through Access. That’d be more explicit. Like

%{root: %{lvl1: [%{only_a: :a}, %{only_b: :b}]}}
|> update_in([:root, :lvl1, Access.all()], options)

%{
  root: %{
    lvl1: [%TypeA{shared: "", only_a: ""}, %TypeB{shared: "", only_b: ""}]
  }
}```
1 Like
3

Thanks @mudasobwa, for prompting me to dig further into the decoders

I’ve gone with custom Poison decoders for each of the relevant structs. It’s really simple to just add the possible structs to the children_types in the decode function and it catches them all on the first pass instead of needing to rewalk the object until all the structs have been matched.

defmodule TypeA do
  defstruct shared: "", only_a: ""
end

defmodule TypeB do
  defstruct shared: "", only_b: ""
end

defmodule Parent do
  defstruct name: "", children: []
end

defimpl Poison.Decoder, for: Parent do
  def decode(value, options) do
    children_types = fn
      %{only_a: _a} -> %TypeA{}
      %{only_b: _b} -> %TypeB{}
    end

    value
    |> Map.update!(:children, fn children ->
      children
      |> Enum.map(
        &Poison.Decode.transform(
          &1,
          Map.put(options, :as, children_types)
        )
      )
    end)
  end
end

"""
{ 
  "name": "parent",
  "children": [
    {
      "shared": "value",
      "only_a": "A"
    }, 
    {
      "shared": "value",
      "only_b": "B"
    }
  ]
}
"""
|> Poison.decode!(keys: :atoms!, as: %Parent{})

%Parent{
  name: "parent",
  children: [
    %TypeA{shared: "value", only_a: "A"}, 
    %TypeB{shared: "value", only_b: "B"}
  ]
}