Validation 使用shoulda匹配器测试唯一性范围验证

我有一个模特

class ObjectiveConcept < ActiveRecord::Base
  validates :objective_id, presence: true
  validates :concept_id, presence: true
  validates :concept_id, uniqueness: {scope: :objective_id}

  belongs_to :objective
  belongs_to :concept
end

当我运行唯一性规范时，我得到以下错误

 ObjectiveConcept#Validations should validate uniqueness of concept_id scoped to objective_id
 Failure/Error: should validate_uniqueness_of(:concept_id).scoped_to(:objective_id)
 ActiveRecord::StatementInvalid:
   PG::Error: ERROR:  null value in column "objective_id" violates not-null constraint
   DETAIL:  Failing row contains (20, null, 0, 2014-09-09 10:24:36.87583, 2014-09-09 10:24:36.87583).
   : INSERT INTO "objective_concepts" ("concept_id", "created_at", "objective_id", "updated_at") VALUES ($1, $2, $3, $4) RETURNING "id"

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我已按照“注意事项”部分的规定填写了唯一性验证的主题，但无效。我如何解决这个问题

通过从主题块中删除代码来修复它

取代

subject { FactoryGirl.create(:object_concept, concept_id: concept.id, objective_id: objective.id) } 

与

subject { FactoryGirl.create(:object_concept, concept_id: concept.id, objective_id: objective.id) } 

FactoryGirl.create(:object_concept, concept_id: concept.id, objective_id: objective.id)