Elixir: best practice to extract data from nested structs

Question

In Elixir we can get data from nested data structures using

data = %{field: %{other_field: 1}}
data[:field][:other_field]

If it contains lists it also can be done using

data = %{field: %{other_field: [1]}}
get_in data, [:field, :other_field, Access.at(0)]

But how to get that data given that data.field.other_field is a structure? Both of the above would fail because structs don't implement Access.fetch/2.

data = %{field: %{other_field: %Struct{a: 1}}}

So what's the right way to access nested structs data other than pattern matching?

Seth · Accepted Answer · 2023-11-21 15:56:22Z

23

Use Access.key/2:

key(key, default \\ nil)

Accesses the given key in a map/struct.

Uses the default value if the key does not exist or if the value being accessed is nil.

iex(1)> defmodule Struct do
...(1)>   defstruct [:a]
...(1)> end
iex(2)> data = %{field: %{other_field: %Struct{a: 1}}}
%{field: %{other_field: %Struct{a: 1}}}
iex(3)> get_in data, [:field, :other_field, Access.key(:a)]
1
iex(4)> get_in data, [:field, :other_field, Access.key(:b, :default)]
:default

edited Nov 21, 2023 at 15:56

Seth

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answered Jul 14, 2016 at 4:08

Dogbert

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Comments

ddengler · Accepted Answer · 2020-11-13 13:08:13Z

2

I use my implementation of ruby's try operator for this.

@spec try(Access.t(), nonempty_list(node) | atom, term) :: term
def try(data, keys, default \\ nil)

def try(nil, _keys, default), do: default

def try(data, keys, default) when is_atom(keys) do
  try(data, [keys], default)
end

def try(data, keys, default) when length(keys) == 1 do
  get_in(data, [Access.key(hd(keys), default)])
end

def try(data, keys, default) do
  get_in(data, [Access.key(hd(keys))])
  |> try(tl(keys), default)
end

Usage in your example

try(data, [:field, :other_field, :a]) # with default nil

try(data, [:field, :other_field, :a], 42) # with default 42

try(data, :single_field_as_atom) # with a single value

edited Nov 13, 2020 at 13:08

answered Aug 22, 2020 at 10:55

ddengler

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Comments

cmo · Accepted Answer · 2022-03-03 02:06:12Z

0

A generic implementation is provided in the Accessible and StructAccess libraries.

This allows you to use the familiar Map syntax.

data = %Data{field: %{other_field: 1}}
data[:field][:other_field]

answered Mar 3, 2022 at 2:06

cmo

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