通过关联创建对象两次使用 has_many 嵌套属性

Question

我的 Rails API 应用程序通过 project_memberships 表在用户和项目之间建立了多对多关系。

型号：

class User < ActiveRecord::Base
  has_many :project_memberships, dependent: :destroy
  has_many :projects, -> { uniq }, through: :project_memberships

  accepts_nested_attributes_for :project_memberships, allow_destroy: true
end

class Project < ActiveRecord::Base
  has_many :project_memberships, dependent: :destroy
  has_many :users, -> { uniq }, through: :project_memberships
end

class ProjectMembership < ActiveRecord::Base
  belongs_to :user
  belongs_to :project

  validates :user, presence: true
  validates :project, presence: true
end

控制器：

class UsersController < ApplicationController
  expose(:user, attributes: :user_params)

  respond_to :json

  # removed unrelated actions

  def update
    user.update user_params
    respond_with user
  end

  private

  def user_params
    params.require(:user).permit(
      :first_name, :last_name,
      project_memberships_attributes: 
        [:id, :project_id, :membership_starts_at, :_destroy]
    )
  end
end

问题是当我使用以下 json 向http://localhost:3000/users/1 发送PUT 请求时：

{"user":{"project_memberships_attributes":[{"project_id": 1}]}

user.update user_params 创建 2 个具有相同 user_id 和 project_id 的 ProjectMembership 记录。

  SQL (0.3ms)  INSERT INTO "project_memberships" ("project_id", "user_id", "created_at", "updated_at") VALUES ($1, $2, $3, $4) RETURNING "id"  [["project_id", 1], ["user_id", 1], ["created_at", "2016-03-18 18:00:07.670012"], ["updated_at", "2016-03-18 18:00:07.670012"]]
  SQL (0.2ms)  INSERT INTO "project_memberships" ("project_id", "user_id", "created_at", "updated_at") VALUES ($1, $2, $3, $4) RETURNING "id"  [["project_id", 1], ["user_id", 1], ["created_at", "2016-03-18 18:00:07.671644"], ["updated_at", "2016-03-18 18:00:07.671644"]]
  (1.0ms)  COMMIT

顺便说一句，通过在嵌套属性中指定 id 来销毁和更新现有记录可以正常工作。

Answer 1

您需要采取的第一步是确保数据库级别的唯一性：

class AddUniquenessConstraintToProjectMemberships < ActiveRecord::Migration
  def change
    # There can be only one!
    add_index :project_memberships, [:user, :project], unique: true
  end
end

这避免了如果我们单独依赖 ActiveRecord 会发生的竞争条件。

来自Thoughtbot: The Perils of Uniqueness Validations。

然后，您希望添加应用程序级别的验证，以避免在违反约束时出现丑陋的 DB 驱动程序异常：

class ProjectMembership < ActiveRecord::Base
  belongs_to :user
  belongs_to :project
  validates :user, presence: true
  validates :project, presence: true 
  validates_uniqueness_of :user_id, scope: :project_id
end

然后，您可以删除关联中的 -> { uniq } lambda，因为您已采取适当的步骤来确保唯一性。

您的其余问题是由于对 accepts_nested_attributes_for 工作原理的误解：

对于每个没有 id 键的散列，一个新记录将是 实例化，除非哈希还包含一个 _destroy 键 计算结果为真。

因此，如果您没有适当的唯一性验证，{"user":{"project_memberships_attributes":[{"project_id": 1}]} 将始终创建新记录。