Question

我正在使用两种模式构建和测试关联：用户，帖子。基本上，用户has_many帖子，但帖子belongs_to只有一个用户。

那就是说，我似乎无法让User.first.posts.build工作。我一直在回错。

**作为一个附带问题，为什么User_id在Post模型中大写？在我见过的大多数例子中，情况并非如此。如果没有大写，它会运行此错误：

Post.create(comment: "yolo molo tolo", user_id: 1)
ActiveRecord::UnknownAttributeError: unknown attribute: user_id

CODE

（1）user.rb

class User < ActiveRecord::Base
  validates :username, presence: true, length: { minimum: 6, maximum: 40}
  has_many :posts 
end

（1）create_users.rb

class CreateUsers < ActiveRecord::Migration
  def change
    create_table :users do |t|
      t.string :username
      t.timestamps
    end
  end
end

（3）Post.rb

class Post < ActiveRecord::Base
  belongs_to :User
  validates :comment, presence: true, length: { minimum: 5, maximum: 30 }
end

（4）create_posts.rb

class CreatePosts < ActiveRecord::Migration
  def change
    create_table :posts do |t|
      t.string :comment
      t.references :User, index: true

      t.timestamps
    end
  end
end

错误

p3 = User.first.posts.new
  User Load (0.2ms)  SELECT "users".* FROM "users" ORDER BY "users"."id" ASC LIMIT 1
ActiveRecord::UnknownAttributeError: unknown attribute: user_id
    from /Users/MightyMouse/.rvm/gems/ruby-2.0.0-p353/gems/activerecord-4.0.3/lib/active_record/attribute_assignment.rb:47:in `rescue in _assign_attribute'
    from /Users/MightyMouse/.rvm/gems/ruby-2.0.0-p353/gems/activerecord-4.0.3/lib/active_record/attribute_assignment.rb:42:in `_assign_attribute'
    from /Users/MightyMouse/.rvm/gems/ruby-2.0.0-p353/gems/activerecord-4.0.3/lib/active_record/attribute_assignment.rb:29:in `block in assign_attributes'
    from /Users/MightyMouse/.rvm/gems/ruby-2.0.0-p353/gems/activerecord-4.0.3/lib/active_record/attribute_assignment.rb:23:in `each'
    from /Users/MightyMouse/.rvm/gems/ruby-2.0.0-p353/gems/activerecord-4.0.3/lib/active_record/attribute_assignment.rb:23:in `assign_attributes'
    from /Users/MightyMouse/.rvm/gems/ruby-2.0.0-p353/gems/activerecord-4.0.3/lib/active_record/associations/association.rb:178:in `initialize_attributes'
    from /Users/MightyMouse/.rvm/gems/ruby-2.0.0-p353/gems/activerecord-4.0.3/lib/active_record/associations/association.rb:251:in `block in build_record'
    from /Users/MightyMouse/.rvm/gems/ruby-2.0.0-p353/gems/activerecord-4.0.3/lib/active_record/core.rb:187:in `initialize'
    from /Users/MightyMouse/.rvm/gems/ruby-2.0.0-p353/gems/activerecord-4.0.3/lib/active_record/inheritance.rb:27:in `new'
    from /Users/MightyMouse/.rvm/gems/ruby-2.0.0-p353/gems/activerecord-4.0.3/lib/active_record/inheritance.rb:27:in `new'
    from /Users/MightyMouse/.rvm/gems/ruby-2.0.0-p353/gems/activerecord-4.0.3/lib/active_record/reflection.rb:189:in `build_association'
    from /Users/MightyMouse/.rvm/gems/ruby-2.0.0-p353/gems/activerecord-4.0.3/lib/active_record/associations/association.rb:250:in `build_record'
    from /Users/MightyMouse/.rvm/gems/ruby-2.0.0-p353/gems/activerecord-4.0.3/lib/active_record/associations/collection_association.rb:114:in `build'
    from /Users/MightyMouse/.rvm/gems/ruby-2.0.0-p353/gems/activerecord-4.0.3/lib/active_record/associations/collection_proxy.rb:229:in `build'
    from (irb):19
    from /Users/MightyMouse/.rvm/gems/ruby-2.0.0-p353/gems/railties-4.0.3/lib/rails/commands/console.rb:90:in `start'
    from /Users/MightyMouse/.rvm/gems/ruby-2.0.0-p353/gems/railties-4.0.3/lib/rails/commands/console.rb:9:in `start'
    from /Users/MightyMouse/.rvm/gems/ruby-2.0.0-p353/gems/railties-4.0.3/lib/rails/commands.rb:62:in `<top (required)>'
    from bin/rails:4:in `require'

Answer 1

帖子模型缺少user_id属性，至少这是你的错误意味着什么。

rails g migration add_user_to_posts user:belongs_to
rake db:migrate

您的迁移文件显示您在posts表中有用户ID，但如果您没有运行迁移，则db中可能会丢失它。

作为快速解决方案，如果您不关心数据库中的数据，我会运行：

rake db:reset

或者

rake db:drop db:create db:migrate

为什么没有.build在rails控制台中工作？

1 个答案: