在插入到多对多表中时，Elixir/Ecto为null FK

# 定义多对多关系表模型
defmodule User do
  use Ecto.Schema

  schema "users" do
    field :name, :string
    has_many :user_roles, UserRole
    many_to_many :roles, Role, join_through: UserRole
  end
end

defmodule Role do
  use Ecto.Schema

  schema "roles" do
    field :name, :string
    many_to_many :users, User, join_through: UserRole
  end
end

defmodule UserRole do
  use Ecto.Schema

  schema "user_roles" do
    belongs_to :user, User
    belongs_to :role, Role
  end
end

# 创建用户和角色
user = %User{name: "John"}
role = %Role{name: "Admin"}

# 创建用户角色关联数据
user_role = %UserRole{user: user, role: role}

# 插入数据
Ecto.Multi.new()
|> Ecto.Multi.insert(:user, user)
|> Ecto.Multi.insert(:role, role)
|> Ecto.Multi.insert(:user_role, user_role)
|> Repo.transaction()
|> Repo.run()

# 注意：在实际应用中，应该根据具体情况进行错误处理和数据验证