Replace item in an array in elixir

Question

Let's say I have a map that looks like this

An organization has an array of properties and the properties has an array of "access_tokens"

organization = %{
  name: "Org",
  properties: [
    %{
      name: "prop1",
      access_tokens: [%{id: "at-1", name: "one-1"}, %{id: "at-2", name: "one-2"}]
    },
    %{
      name: "prop2",
      access_tokens: [%{id: "at-3", name: "two-1"}, %{id: "at-4", name: "two-2"}]
    }
  ]
}

Now I have this map, for one particular access token, the id for this one matches one of the access tokens for the 2nd property:

access_token = %{id: "at-3", name: "new name"}

What is the best way to iterate over to update the organization%{} map with the new access token? What I want to do is find the access token that matches based on id and replace it with the new one that has the new name.

I have some round-about ways of doing this, but what is a clean way to do this in Elixir.

Aleksei Matiushkin · Accepted Answer · 2019-11-12 16:04:14Z

15

You do not need to iterate, just use Access, Access.filter/1, and Access.all/0:

put_in(
  organization,
  [:properties,
   Access.all(),
   :access_tokens,
   Access.filter(&match?(%{id: "at-3"}, &1)),
   :name],
  "new_name")

I always wonder, how this one of the most powerful things in the language is extremely underrated.

answered Nov 12, 2019 at 16:04

Aleksei Matiushkin

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7 Comments

Aleksei Matiushkin Over a year ago

@7stud No one single language I am aware of provides such an elegant way of accessing deeply nested elements with an ability to filter them on each level and there is a great documentation available. Note, that if prop1 also had the matched element, it’d be updated too.

7stud Over a year ago

there is a great documentation -- Then perhaps you could explain why you always use put_in() instead of update_in(), which would seem to be the more logical choice.

dylanjha Over a year ago

Super cool, thank you! That is so useful! That works, but it looks like it's not working if "at-3" is a variable

put_in(organization,[:properties, Access.all(), :access_tokens, Access.filter(&match?(%{id: access_token.id}, &1)), :name], "new_name")

Is there a way to get around that?

Brett Beatty Over a year ago

I'm not sure where I would find this in the docs, but it seems that the Elixir convention is to use put anywhere you want to say "this key is now associated with this value" and an update anywhere you want to say "I want to do this operation on the value associated with this key (increment a counter, prepend to a list, etc.)". In this example, put_in makes more sense because we want name: "new_name" in our map. We would use update_in if we wanted a pair more like name: "two-1-new_name". Essentially I'd use put_in anywhere you'd throw out the argument to the update_in callback.

Brett Beatty Over a year ago

@dylanjha Where match?(pattern, expr) is a macro, it won't like trying to match on your new_at.id. You could save that as a variable and pin it: &match?(%{id: ^new_id}, &1), or you could filter with a simpler function: & &1 == new_at.id.

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