Elixir comprehension returning a star character '*'

问题

I have a list of Persona models being returned in p.followings and I want to extract the followed_id field from this list of models into a separate list.

p.followings

returns...

[
  %Poaster.Personas.Following{
    __meta__: #Ecto.Schema.Metadata<:loaded, "followings">,
    followed: %Poaster.Personas.Persona{
      __meta__: #Ecto.Schema.Metadata<:loaded, "personas">,
      background_image_url: nil,
      bio: "ASDF",
      followings: #Ecto.Association.NotLoaded<association :followings is not loaded>,
      id: 42,
      inserted_at: ~N[2020-08-14 01:52:17],
      name: nil,
      profile_image_url: nil,
      updated_at: ~N[2020-08-14 16:19:56],
      user: #Ecto.Association.NotLoaded<association :user is not loaded>,
      user_id: 1,
      username: "test"
    },
    followed_id: 42,
    id: 1,
    inserted_at: ~N[2020-08-12 20:35:09],
    persona: #Ecto.Association.NotLoaded<association :persona is not loaded>,
    persona_id: 1,
    updated_at: ~N[2020-08-12 20:35:09]
  }
]

I simply want to get a list of the followed_id here so I can make a query to get a list of posts from those personas I am following.

I want to get back something like [42].

When I do Enum.map(ps.followings, fn follow -> follow.followed_id end), which is what I expected to be able to run to get this, I am getting back in the console just '*'

When I tried to use a comprehension with the into option, into an empty list, this is also what I got.

persona_ids = []
for p <- p.followings, into: persona_ids, do: p.followed_id
IO.inspect(persona_ids)
[]

However, when I run the above comprehension with p.followed, it returns a list of the Personas:

for p <- p.followings, into: persona_ids, do: p.followed   
[
  %Poaster.Personas.Persona{
    __meta__: #Ecto.Schema.Metadata<:loaded, "personas">,
    background_image_url: nil,
    bio: "ASDF",
    followings: #Ecto.Association.NotLoaded<association :followings is not loaded>,
    id: 42,
    inserted_at: ~N[2020-08-14 01:52:17],
    name: nil,
    profile_image_url: nil,
    updated_at: ~N[2020-08-14 16:19:56],
    user: #Ecto.Association.NotLoaded<association :user is not loaded>,
    user_id: 1,
    username: "test"
  }
]

I need the list of IDs, not the list of Persona models, so that I can make an appropriate Ecto query to get the posts from the Personas I follow.

What is going on here? What am I doing wrong? Is there a better way to do what I am trying to do?

回答1:

As I mentioned in the comment, and as discussed on this other post, the '*' you're receiving is in fact the list you expect: [42].

This happens because 42 is the codepoint of the * character (you can verify this by doing ?* in an iex session). In Elixir and Erlang, when you have a list of integers and all of the integers are valid codepoints for characters, it will print the charlist when you use IO.inspect, but it is a list and you can use it like you would use any list.

For instance, if you type [104, 101, 108, 108, 111] into the iex prompt, you will get back 'hello', but the single quotes denote that it's a charlist, and you can perform any list operations you'd like on it.

来源：https://stackoverflow.com/questions/63473722/elixir-comprehension-returning-a-star-character