Active Record如何通过rails中的关联将记录添加到has_many：

Question

I have quite the Problem with my has_many :through association. The models look like this:

class User < ActiveRecord::Base
  has_many :roles
  has_many :datasets, through: :roles
  has_secure_password
end

class Role < ActiveRecord::Base
  belongs_to :user
  belongs_to :dataset
end

class Dataset < ActiveRecord::Base
  has_many :roles
  has_many :users, through: :roles
end

I want to add a record to roles every time a new Dataset is created. It should be created with the new Dataset, an existing User 'Admin' and the column name of Role should be set to 'Admin'.

I tried everything I found on Stackoverflow but nothing works for me.

My create method in the DatasetController looks this:

def create
    @dataset = Dataset.new(dataset_params)
    @dataset.save
    @user = User.find_by(name: 'Admin')
    @dataset.users << @user
    #@dataset.roles.create(user: @user, dataset: @dataset, name: 'Admin')    
    respond_with(@dataset)
  end

I tried both the << operator and the create method.

the first results in:

ActiveRecord::UnknownAttributeError in DatasetsController#create
unknown attribute: dataset_id

the second in:

ActiveModel::MissingAttributeError in DatasetsController#create
can't write unknown attribute `user_id

Does anyone know why I get these errors?

my schema.rb:

create_table "datasets", force: true do |t|
  t.string   "name"
  t.text     "description"
  t.datetime "created_at"
  t.datetime "updated_at"
end

create_table "roles", force: true do |t|
  t.string   "name"
  t.datetime "created_at"
  t.datetime "updated_at"
end

create_table "users", force: true do |t|
  t.string   "name"
  t.string   "mail"
  t.string   "password_digest"
  t.datetime "created_at"
  t.datetime "updated_at"
end

Answer 1

Your Role model needs columns to refer to which User and Dataset it belongs. Without these it has no idea who belongs to who.

So you simply need to create a migration to add these columns:

class AddRefererColumnsToRole < ActiveRecord::Migration
  def change
    add_column :roles, :user_id, :integer
    add_column :roles, :dataset_id, :integer
  end
end