How to write DRY spec for verifying requst parameters

Question

I write spec for controllers in rails application. I want to write DRY spec for verifying request parameters check, rspec can be helpful to use shared_example_for. For example.

shared_example_for "verifying request parameters" do
  it "should return success for required parameter" do
    get @action, @required_parameters
    response.should be_success
  end

  it "should return error for lack of required parameters" do 
    get @action, @lack_of_parameters
    response.should 400
  end 
end

describe BlogController do
  fixtures :blogs
  describe "show" do
    before(:each) do
      @action = "show"
      @required_parameters = {:month=>blogs(:one).month, :day=>blogs(:one).day}
      @lack_of_parameters = {:month=>blogs(:one).month}
    end

    it_should_behave_like "verifying request parameters"
  end

  describe "edit" do
    before(:each) do
      @action = "edit"
      @required_parameters = {:month=>blogs(:one).month, :day=>blogs(:one).day}
      @lack_of_parameters = {:month=>blogs(:one).month}
    end

    it_should_behave_like "verifying request parameters"
  end

end

I want to write more DRY like below

describe BlogController do
  fixtures :blogs

  {
    :show=>{:month=>blogs(:one).month, :day=>blogs(:one).day},
    :edit=>{:month=>blogs(:one).month, :day=>blogs(:one).day}
  }.each |act, prams| do

    describe "blog/#{act}" do
      before(:each) do
        @action = act
        @required_parameters = params
        @lack_of_parameters = params.reject{|k,v| k==:month }
      end

      it_should_behave_like "verifying request parameters"
    end
  end
end

end

This code has problem that "blogs" is not undefined. It is caused by fixtures out of scope on "describe" block. And "lack_of_parameters" is incomplete for lacking pattern. How to solve it or write DRY spec for verifying parameters.