title: Problem with Creation form (Simpleform) PG::NotNullViolation: ERROR: null value in column with Rails
body: <p>Hello i have an issue with my app Rails when i try to create an "Enfant" who belongs to an user and a Nounou but my problem is when i create an "enfant" i'm a user with an ID but, I haven't chosen yet a nounou so i haven't got a nounou_id this is my differents code(i try to put optional: true but it doesn't work : 
  Models and Schema</p>
  
  <pre><code>class Enfant &lt; ApplicationRecord
    belongs_to :user
    belongs_to :nounou, optional: true
  end
  
  
  class Nounou &lt; ApplicationRecord
    has_many :enfants
  end
  
  
  class User &lt; ApplicationRecord
    # Include default devise modules. Others available are:
    # :confirmable, :lockable, :timeoutable, :trackable and :omniauthable
    devise :database_authenticatable, :registerable,
           :recoverable, :rememberable, :validatable
    has_many :enfants
  end
  
  
    create_table "enfants", force: :cascade do |t|
      t.string "last_name"
      t.string "first_name"
      t.bigint "nounou_id", null: false
      t.bigint "user_id", null: false
      t.datetime "created_at", precision: 6, null: false
      t.datetime "updated_at", precision: 6, null: false
      t.index ["nounou_id"], name: "index_enfants_on_nounou_id"
      t.index ["user_id"], name: "index_enfants_on_user_id"
    end
  
    create_table "nounous", force: :cascade do |t|
      t.string "name"
      t.integer "price"
      t.string "localisation"
      t.integer "evaluation"
      t.integer "places"
      t.string "first_name"
      t.string "last_name"
      t.string "photo"
      t.datetime "created_at", precision: 6, null: false
      t.datetime "updated_at", precision: 6, null: false
    end
  
    create_table "users", force: :cascade do |t|
      t.string "email", default: "", null: false
      t.string "encrypted_password", default: "", null: false
      t.string "reset_password_token"
      t.datetime "reset_password_sent_at"
      t.datetime "remember_created_at"
      t.datetime "created_at", precision: 6, null: false
      t.datetime "updated_at", precision: 6, null: false
      t.string "username"
      t.string "photo"
      t.string "first_name"
      t.string "last_name"
      t.index ["email"], name: "index_users_on_email", unique: true
      t.index ["reset_password_token"], name: "index_users_on_reset_password_token", unique: true
    end
  
  end
  
  
  </code></pre>