Elixir 添加对现有列和表的引用

以下是我的原始迁移：

def change do
  create table(:clipboard_items) do
    add(:clipboard_id, :integer)

    timestamps()
  end
end

在这次迁移过程中，我忘记为每个字段添加引用。应该是这样的：

def change do
  create table(:clipboard_items) do
    add(:clipboard_id, references(:clipboards))

    timestamps()
  end
end

如何将引用添加到现有表中

我试过这个：

  def change do
    alter table(:clipboard_items) do
      modify :clipboard_id, references(:clipboards)
    end
  end

但我得到了这个错误：

alter table clipboard_items
** (Postgrex.Error) ERROR 42710 (duplicate_object) constraint "clipboard_items_clipboard_id_fkey" for relation "clipboard_items" already exists

---

这是使我不能级联删除，任何帮助这将是伟大的

结果表明，这将起作用：

defmodule Dreamhouse.Repo.Migrations.AddReferenceToClipboardItem do
  use Ecto.Migration

  def up do
    execute "ALTER TABLE clipboard_items DROP CONSTRAINT clipboard_items_clipboard_id_fkey"

    alter table(:clipboard_items) do
      modify :clipboard_id, references(:clipboards, on_delete: :delete_all)
    end
  end

  def down do
    execute "ALTER TABLE clipboard_items DROP CONSTRAINT clipboard_items_clipboard_id_fkey"

    alter table(:clipboard_items) do
      modify :clipboard_id, references(:clipboards, on_delete: :nothing)
    end
  end
end