Say I have the following models

import Ecto.Changeset
import Ecto.Schema

defmodule Parent do
  schema "parents" do
    has_one :child, Child
  end

  def changeset(%Parent{} = ch, attrs) do
    ch
    |> cast(attrs, [])
  end
end

defmodule Child do
  schema "childs" do
    belongs_to :parent, Parent
  end

  def changeset(%Child{} = ch, attrs) do
    ch
    |> cast(attrs, [:parent_id])
    |> foreign_key_constraint(:parent_id)
  end
end

Whenever I create a Parent, I want a Child to be created and associated with the Parent automatically. Whenever I update Parent, I only want the Child to be updated if it is a part of the attrs or changeset given to Parent.changeset/2. Otherwise, the changeset should silently work.

Basically, Child should only be part of the changeset if it is being updated or if Parent is being created.

I’ve tried playing around with cast_assoc and put_assoc but neither seems to work the way I want it to.