20170622103130_create_psq_answer.exs

defmodule Playground.Repo.Migrations.CreatePlayground.Psq.Answer do
  use Ecto.Migration

  def change do
    create table(:psq_answers) do
      add :survey_id, :integer
      timestamps()
    end

  end
end

20170622124351_create_anwer_detail.exs

defmodule Playground.Repo.Migrations.CreatePlayground.AnswerDetail do
  use Ecto.Migration

  def change do
    create table(:psq_answer_details) do
      add :question_id, :integer
      add :psq_answer_id, references(:psq_answers)
      add :question_type, :string
      add :content, :string

      timestamps()
    end

  end
end

answer.ex

defmodule Playground.Psq.Answer do
  use Ecto.Schema
  import Ecto.Changeset
  alias Playground.Psq.Answer


  schema "psq_answers" do
    field :survey_id, :integer

    has_many :psq_answer_details, Playground.Psq.AnswerDetail

    timestamps()
  end

  @doc false
  def changeset(%Answer{} = answer, attrs) do
    answer
    |> cast(attrs, [:survey_id])
    |> validate_required([:survey_id])
  end
end

answer_detail.ex

defmodule Playground.Psq.AnswerDetail do
  use Ecto.Schema
  import Ecto.Changeset
  alias Playground.Psq.AnswerDetail


  schema "psq_answer_details" do
    field :content, :string
    field :question_id, :integer
    field :question_type, :string

    belongs_to :psq_answer, Playground.Psq.Answer

    timestamps()
  end

  @doc false
  def changeset(%AnswerDetail{} = answer_detail, attrs) do
    answer_detail
    |> cast(attrs, [:question_id, :question_type, :content])
    |> validate_required([:question_id, :question_type])
  end
end

insert.exs

  import Ecto.Query, warn: false
  alias Playground.Repo

  alias Playground.Psq.Answer
  alias Playground.Psq.AnswerDetail

  Repo.insert!(%Answer{survey_id: 1, psq_answer_details: [%AnswerDetail{question_id: 1, question_type: "fill", content: "fff"}]})

result

in psq_answers table:

id | survey_id
1  |         1

in psq_answer_details table:

id | question_id | psq_answer_id | question_type | content
 1 |           1 |    NULL       | fill          | fff
 
 no psq_answer_id inserted