我有一个相当复杂的模型设置：
class LineItem < ActiveRecord::Base # STI parent class
end
class VendorLineItem < LineItem
belongs_to: :unit_rate, class_name: 'Billing::UnitRate'
has_one :contract_resource, through: :unit_rate
has_one :resource, through: :contract_resource
end
请注意，以上两个类都没有命名空间，而下面的关联模型都存在于Billing命名空间中。
class Billing::UnitRate < ActiveRecord::Base
belongs_to :contract_resource, class_name: 'Billing::ContractResource'
has_one :contract, through: :contract_resource
has_one :resource, through: :contract_resource
has_many :vendor_line_items, dependent: :destroy
end
class Billing::ContractResource < ActiveRecord::Base
belongs_to :contract
belongs_to :resource, class_name: 'Billing::Resource'
has_many :unit_rates, dependent: :destroy
end
class Billing::Resource < ActiveRecord::Base
has_many :contract_resources
has_many :contracts, through: :contract_resources
has_many :unit_rates, through: :contract_resources
end
我的问题出在VendorLineItem课堂上，我需要在其中找到相关的resource.
如果我正在处理持久数据，一切都很好。在 Rails 控制台中：
2.2.3 :001 > x = VendorLineItem.last
VendorLineItem Load (9.8ms) SELECT "line_items".* FROM "line_items" WHERE "line_items"."type" IN ('VendorLineItem') ORDER BY "line_items"."id" DESC LIMIT 1
=> #<VendorLineItem id: 42, fire_id: 938774, type: "VendorLineItem", unit_rate_id: 716, units_utilized: #<BigDecimal:6184d30,'0.1E1',9(18)>, fire_department_id: nil, dollar_amount: nil, created_at: "2016-11-18 16:22:23", updated_at: "2016-11-18 16:22:23", total: #<BigDecimal:6184060,'0.25E2',9(18)>>
2.2.3 :002 > x.resource
Billing::Resource Load (8.9ms) SELECT "billing"."resources".* FROM "billing"."resources" INNER JOIN "contracts_resources" ON "billing"."resources"."id" = "contracts_resources"."resource_id" INNER JOIN "billing"."unit_rates" ON "contracts_resources"."id" = "billing"."unit_rates"."contract_resource_id" WHERE "billing"."unit_rates"."id" = $1 LIMIT 1 [["id", 716]]
=> #<Billing::Resource id: 39, name: nil, description: nil, code: nil, sort_order: nil, is_active: true, constant_name: "E0", created_at: "2015-10-14 15:59:00", updated_at: "2015-10-14 15:59:00", vendor_id: 1, resource_type_id: 37>
如果，OTOH，我正在使用一个新实例，则resource返回为零（rails 甚至没有尝试；没有发出 sql）。
2.2.3 :003 > vli = VendorLineItem.new unit_rate_id: 711
=> #<VendorLineItem id: nil, fire_id: nil, type: "VendorLineItem", unit_rate_id: 711, units_utilized: nil, fire_department_id: nil, dollar_amount: nil, created_at: nil, updated_at: nil, total: nil>
2.2.3 :004 > vli.resource
=> nil
但是我可以通过显式的关联链访问资源：
2.2.3 :005 > vli.unit_rate.contract_resource.resource
Billing::UnitRate Load (8.9ms) SELECT "billing"."unit_rates".* FROM "billing"."unit_rates" WHERE "billing"."unit_rates"."id" = $1 LIMIT 1 [["id", 711]]
Billing::ContractResource Load (8.8ms) SELECT "contracts_resources".* FROM "contracts_resources" WHERE "contracts_resources"."id" = $1 LIMIT 1 [["id", 261]]
Billing::Resource Load (8.7ms) SELECT "billing"."resources".* FROM "billing"."resources" WHERE "billing"."resources"."id" = $1 LIMIT 1 [["id", 34]]
=> #<Billing::Resource id: 34, name: nil, description: nil, code: nil, sort_order: nil, is_active: true, constant_name: "D5", created_at: "2015-10-14 15:59:00", updated_at: "2015-10-14 15:59:00", vendor_id: 1, resource_type_id: 19>
我也可以has_one :contract_resource, through: :unit_rate毫无问题地通过关联找到资源：
2.2.3 :007 > vli.contract_resource.resource
Billing::ContractResource Load (8.7ms) SELECT "contracts_resources".* FROM "contracts_resources" INNER JOIN "billing"."unit_rates" ON "contracts_resources"."id" = "billing"."unit_rates"."contract_resource_id" WHERE "billing"."unit_rates"."id" = $1 LIMIT 1 [["id", 711]]
Billing::Resource Load (8.7ms) SELECT "billing"."resources".* FROM "billing"."resources" WHERE "billing"."resources"."id" = $1 LIMIT 1 [["id", 34]]
=> #<Billing::Resource id: 34, name: nil, description: nil, code: nil, sort_order: nil, is_active: true, constant_name: "D5", created_at: "2015-10-14 15:59:00", updated_at: "2015-10-14 15:59:00", vendor_id: 1, resource_type_id: 19>
为什么has_one: resource, through: :contract_resource协会在这里不起作用？