Postgresql 由于外键约束无法删除对象

我试图做一个简单的user.destroy，但遇到了以下错误:

错误:表"用户"上的更新或删除违反了表"标识"上的外键约束"fk_rails_5373344100"

这是我的身份迁移

class CreateIdentities < ActiveRecord::Migration
  def change
    create_table :identities do |t|
      t.references :user, index: true, foreign_key: true
      t.string :provider
      t.string :uid

      t.timestamps null: false
    end
  end
end

以下是我的用户和身份模型:

class Identity < ActiveRecord::Base
  belongs_to :user

  validates_presence_of :uid, :provider
  validates_uniqueness_of :uid, :scope => :provider

  def self.find_for_oauth(auth)
    find_or_create_by(uid: auth.uid, provider: auth.provider)
  end
end

和用户:

class User < ActiveRecord::Base
  TEMP_EMAIL_PREFIX = 'ricky@writeit.com'
  TEMP_EMAIL_REGEX = /\ricky@writeit.com/

  # Include default devise modules. Others available are:
  # :lockable, :timeoutable
  devise :database_authenticatable, :registerable, :confirmable,
         :recoverable, :rememberable, :trackable, :validatable, :omniauthable

  validates_format_of :email, :without => TEMP_EMAIL_REGEX, on: :update
...

end

我不熟悉外键和引用，所以我根本不知道如何修复它.