问如何用唯一的“attendable_type”和“attendable_id”- rails 4在Rails中索引多态表

EN

class CreateAttendances < ActiveRecord::Migration
  def change
    create_table :attendances do |t|
      t.references :attendable, polymorphic: true, index: true
      t.references :user, index: true
      t.timestamps null: false
    end
  end
end

  create_table "attendances", force: :cascade do |t|
    t.integer  "attendable_id"
    t.string   "attendable_type"
    t.integer  "user_id"
    t.datetime "created_at",      null: false
    t.datetime "updated_at",      null: false
  end

  add_index "attendances", ["attendable_type", "attendable_id"], name: "index_attendances_on_attendable_type_and_attendable_id"
  add_index "attendances", ["user_id"], name: "index_attendances_on_user_id"

class AddUniqToAttendances < ActiveRecord::Migration
  def change
    add_index :attendances, [:attendable_id, :attendable_type], :unique => true
  end
end

== 20170126154413 AddUniqToAttendances: migrating =============================
-- add_index(:attendances, [:attendable_id, :attendable_type], {:unique=>true})
rake aborted!
StandardError: An error has occurred, this and all later migrations canceled:

SQLite3::ConstraintException: UNIQUE constraint failed: attendances.attendable_id, attendances.attendable_type: CREATE UNIQUE INDEX "index_attendances_on_attendable_id_and_attendable_type" ON "attendances" ("attendable_id", "attendable_type")

add_index "attendances", ["attendable_type", "attendable_id"], name: "index_attendances_on_attendable_type_and_attendable_id"

add_index "attendances", ["attendable_type", "attendable_id"], name: "index_attendances_on_attendable_type_and_attendable_id", unique: true