It does do that - if it is passed in as place_id it is validated with assoc_constraint; otherwise it is checked with cast_assoc required: true.

However the code as listed is really not complete because it does not handle updates to the record that do not include a change to the place; e.g. record is loaded from the db, place_id is set, but because place_id is not changed cast_assoc fails on it. In my project I’m using a slightly modified version that will pass the changeset if it contains the key (place_id) already:

    def cast_or_constraint_assoc(changeset, name) do
      {:assoc, %{owner_key: key}} = changeset.types[name]
      #assoc id was directly set? confirm its valid
      if changeset.changes[key] do
        assoc_constraint(changeset, name)
      else
        #assoc key is already present, and not changed? do nothing
        if Map.get(changeset.data, key) do
          changeset
        else
          #we need to insert a new assoc (or errors)
          cast_assoc(changeset, name, required: true)
        end
      end
    end

I think you want the following given your schema:

def changeset(struct, params \\ %{}) do
  struct
  |> cast(params, [:rating, :place_id])
  |> validate_required([:rating]
  |> assoc_constraint(:place)
end

For a :belongs_to association, use assoc_constraint/3 for validation. It let’s Ecto check whether the Place to which the rating belongs exists. cast_assoc/3 would go on the Place schema to check Rating. Don’t use validate_required/3 to check association constraints (as instructed here).

I struggled with the different changeset validations in the same context. I outlined my findings here: Ecto Association vs Foreign Key Constraints

I understsand now, thank you. @wojtekmach sounds like a good way to go about this.

Don’t cast_assoc accept a required: true option?

For people looking for more validation, the code below works pretty well for me.
Using this code helps when you want to deal with a new Map or an existing Struct or an existing id was given for the association…

def put_or_cast_or_constraint_assoc(changeset, name) do
  {:assoc, %{owner_key: key, related: type}} = changeset.types[name] |> IO.inspect

  if changeset.changes[key] do
    assoc_constraint(changeset, name)
  else
    case val = changeset.params[name |> Atom.to_string] do
      %correct_type{} when correct_type == type ->
        put_assoc(changeset, name, val)
      %wrong_type{} ->
        add_error(changeset, name, "Wrong struct given to #{key}", given: wrong_type, wanted: type)
      %{} ->
        cast_assoc(changeset, name, required: true)
      _ ->
        add_error(changeset, name, "No valid #{name} nor #{key}")
    end
  end
end