Hello,

Why is that my update function nillify the association entity.

I have two entities: Post and User

So create function controller is like this:

def create(conn, %{"post" => post_params}) do
  current_user = get_session(conn, :current_user)
  post_params = Map.merge(%{"user" => current_user}, post_params)
  case Blog.create_post(post_params) do
    {:ok, post} ->
      conn
       |> put_flash(:info, "Post created successfully.")
       |> redirect(to: Routes.post_path(conn, :show, post))
    {:error, %Ecto.Changeset{} = changeset} ->
      render(conn, "new.html", changeset: changeset)
  end
end

which associates the user to the post.

Post schema

def changeset(post, attrs) do
    post
    |> cast(attrs, [:title, :content, :is_published])
    |> validate_required([:title, :content])
    |> put_slug()
    |> put_assoc(:user, attrs["user"])
end

My problem is when updating a post the user associated with the post gets nillify:

UPDATE "posts" SET "content" = $1, "user_id" = $2, "updated_at" = $3 WHERE "id" = $4 ["Loading...", nil, ~N[2018-12-02 13:35:59], 4]

controller:

def update(conn, %{"id" => id, "post" => post_params}) do
    post = Blog.get_post!(id)
      
    case Blog.update_post(post, post_params) do
      {:ok, post} ->
        conn
        |> put_flash(:info, "Post updated successfully.")
        |> redirect(to: Routes.post_path(conn, :show, post))

      {:error, %Ecto.Changeset{} = changeset} ->
        render(conn, "edit.html", post: post, changeset: changeset)
    end
  end

My question is how to retain the user associated to the post?

Should I write a new params that Map.merge the post_params and the current_user which is the same as I did in the create function?

Please ask clarification if my explanation is not clear. Thank you.