问Rails: PG::NotNullViolation: ERROR："id“列中的空值违反非空约束

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class User < ApplicationRecord
  belongs_to :role
end

class Role < ApplicationRecord
end

class Admin < ApplicationRecord
  before_save :set_admin_role

  belongs_to :user

  private

  def set_admin_role
    # set role_id to '1' except if role_id is not empty
    user.role_id = '1' if user.role_id.nil?
  end
end

class Student < ApplicationRecord
  before_save :set_student_role

  belongs_to :user

  private

  def set_student_role
    # set role_id to '2' except if role_id is not empty
    user.role_id = '2' if user.role_id.nil?
  end
end

ActiveRecord::NotNullViolation (PG::NotNullViolation: ERROR:  null value in column "role_id" violates not-null constraint)

class CreateUsers < ActiveRecord::Migration[6.0]
  def change
    create_table :users do |t|
      t.string :email
      t.string :password_digest
      t.references :role, null: true, foreign_key: true

      t.timestamps
    end
  end
end

class User < ApplicationRecord
  belongs_to :role, optional: true
end