I have this error when the client asks for an entity with an association, and I didn’t preload the association.

Example:
Client sends query document:

user {
  name
  group {
    id
  }
}

A user optionally belongs to a group.
If I return the User without preloading the group:

def create_user(attrs) do
  %User{}
  |> User.changeset(attrs)
  |> Repo.insert()
end

The following error is returned to the client:

1. locations: [{column: 11, line: 33}]
2. message: “Cannot return null for non-nullable field”
3. path: [“createUser”, “user”, “group”, “id”]

But with preload it will work (even when the user has no group, and thus resolves to null as well):

def create_user(attrs) do
  %User{}
  |> User.changeset(attrs)
  |> Repo.insert()
  |> case do
       {:ok, user} ->
         {:ok, Repo.preload(user, [:group])}
       error ->
         error
     end
end

In the schema, there is no non_null constraint for the group:

  object :user do
    field :group, :user_group
  end

  object :user_group do
    field :id, non_null(:id)
  end

Question: why without preloading do I obtain the error? group can be null anyway from the Absinthe schema, so why does preloading matter?