DataMapper中的关联

Question

在下面的代码中，我遇到了关联问题。

我得到的错误是对最后一行代码的注释。

编辑:我简化了代码.

require 'rubygems'
require 'data_mapper' # requires all the gems listed above
require 'pp'

DataMapper.setup(:default, 'sqlite:///Users/chris/Dropbox/HawkEye-DB Test/store.sqlite')

class Manufacturer
  include DataMapper::Resource
  property :id, Serial
  property :name, String

  has n, :products
end

class Product
  include DataMapper::Resource
  property :id, Serial    
  property :name, String

  belongs_to :manufacturer
  has 1, :productoptionset
end


class Productoptionset
  include DataMapper::Resource
  property :id, Serial    
  property :name, String

  belongs_to :product

end

DataMapper.auto_migrate!



# Make some manufactureres
gortex = Manufacturer.create(:name => 'Gortex')
garmin = Manufacturer.create(:name => 'Garmin')

gps = garmin.products.create(:name => 'GPS Unit')

samegps = Product.get(1)

pp samegps.productoptionset.create # undefined method `create' for nil:NilClass (NoMethodError)

Answer 1

create是一个类方法(类似于Java中的静态方法)，因此不能对实例(在本例中为非实例)调用它:)

您可以像这样创建对象

class Manufacturer
  include DataMapper::Resource
  property :id, Serial
  property :name, String

  has n, :products
end

class Product
  include DataMapper::Resource
  property :id, Serial    
  property :manufacturer_id, Integer
  property :name, String

  belongs_to :manufacturer
  has 1, :productoptionset
end


class Productoptionset
  include DataMapper::Resource
  property :id, Serial  
  property :product_id, Integer
  property :name, String

  belongs_to :product
end

DataMapper.auto_migrate!



# Make some manufactureres
gortex = Manufacturer.create(:name => 'Gortex')
garmin = Manufacturer.create(:name => 'Garmin')

garmin.products << Product.create(:name => 'GPS Unit')

samegps = Product.get(1)

samegps.productoptionset = Productoptionset.create(:name => "MyProductoptionset")

Answer 2

has 1创建一个访问器productoptionset，它最初是nil而不是集合。nil没有方法create。收藏品已经。

您可以通过以下方式创建和关联ProductOptionSet

Productoptionset.create(:name => 'Foo', :product => gps)