Elixir理解返回一个星形字符"*" [英] Elixir comprehension returning a star character '*'

问题描述

我有一个在 p.followings 中返回的Persona模型的列表，我想从该模型列表中将 followed_id 字段提取到一个单独的列表中.

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]
  }
]

我只是想在此处获取followed_id的列表，因此我可以进行查询以获取我所关注的角色的帖子列表.

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.

我想找回 [42] 之类的东西.

当我执行 Enum.map(ps.followings，fn follow-> follow.followed_id end)时，这是我期望能够运行的功能，我回来了在控制台中，只需'*'

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 '*'

当我尝试对 into 选项使用理解时，进入一个空列表，这也是我得到的.

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)
[]

但是，当我对 p.followed 进行上述理解时，它将返回角色的列表:

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"
  }
]

我需要ID列表，而不是Persona模型列表，以便我可以进行适当的Ecto查询以从我关注的Personas中获取帖子.

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.

这是怎么回事?我究竟做错了什么?有没有更好的方法来做我想做的事?

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

推荐答案

正如我在评论中提到的那样，并且在

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].

之所以会这样，是因为42是 * 字符的代码点(您可以通过在iex会话中执行?* 进行验证).在Elixir和Erlang中，当您有一个整数列表并且所有整数都是字符的有效代码点时，使用 IO.inspect 时它将打印字符列表，但这是一个列表，您可以像使用任何列表一样使用它.

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.

例如，如果您在iex提示符下键入 [104、101、108、108、111] ，您将返回'hello'，但单个引号表示它是一个字符列表，您可以对其执行任何列表操作.

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.

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