Access.get/3 中没有函数子句匹配 - 为什么会出现此错误？

Question

当我尝试从数据库中获取列表时出现此错误，我不明白为什么。

这是引发错误的函数：

  def list_lectures do
    Lecture
    |> Repo.all()
    |> Repo.preload(:author [user: :credential])
  end

这是我运行代码时遇到的错误：

iex(1)> Ram.CMS.list_lectures
[debug] QUERY OK source="lectures" db=3.4ms decode=0.7ms queue=0.4ms
SELECT l0."id", l0."source", l0."theme", l0."time", l0."title", l0."author_id", l0."inserted_at", l0."updated_at" FROM "lectures" AS l0 [] 
** (FunctionClauseError) no function clause matching in Access.get/3    

    The following arguments were given to Access.get/3:

        # 1
        :author

        # 2
        [user: :credential]

        # 3
        nil

    Attempted function clauses (showing 5 out of 5):

        def get(%module{} = container, key, default)
        def get(map, key, default) when is_map(map)
        def get(list, key, default) when is_list(list) and is_atom(key)
        def get(list, key, _default) when is_list(list)
        def get(nil, _key, default)

    (elixir) lib/access.ex:265: Access.get/3
    (ram) lib/ram/cms.ex:24: Ram.CMS.list_lectures/0
iex(1)> 

  schema "lectures" do
    field :source, :string
    field :theme, :string
    field :time, :string
    field :title, :string
    belongs_to :author, Author

    timestamps()
  end

  schema "credentials" do
    field :email, :string
    field :password, :string
    belongs_to :user, User

    timestamps()
  end

schema "users" do
    field :name, :string
    field :role, :string
    has_one :credential, Credential

    timestamps()
  end

这是项目链接：https://github.com/DavidNeumark/ram

Answer 1

Access.get/3 文件说：

获取容器中给定键的值（映射、关键字列表或实现访问行为的结构）。

你的第一个参数给 Access.get/3：

:author 不是映射、关键字列表或结构。是原子。

我猜你想预加载嵌套关联，所以它应该是这样的：

  def list_lectures do
    Lecture
    |> Repo.all()
    |> Repo.preload(author: [user: :credential])
  end

更新：检查你的 github 后，lib/ram/cms/lecture.ex 错过了 Author 的别名。它导致上面的代码不起作用

defmodule Ram.CMS.Lecture do
  use Ecto.Schema
  import Ecto.Changeset

  alias Ram.CMS.Author

  schema "lectures" do
    field :source, :string
    field :theme, :string
    field :time, :string
    field :title, :string
    belongs_to :author, Author

    timestamps()
  end

  @doc false
  def changeset(lecture, attrs) do
    lecture
    |> cast(attrs, [:title, :time, :source, :theme])
    |> validate_required([:title, :time, :source, :theme])
  end
end