# File lib/fog/aws/requests/iam/detach_group_policy.rb, line 32
        def detach_group_policy(group_name, policy_arn)
          if policy_arn.nil?
            raise Fog::AWS::IAM::ValidationError, "1 validation error detected: Value null at 'policyArn' failed to satisfy constraint: Member must not be null"
          end

          managed_policy = self.data[:managed_policies][policy_arn]

          unless managed_policy
            raise Fog::AWS::IAM::NotFound, "Policy #{policy_arn} does not exist."
          end

          unless self.data[:groups].key?(group_name)
            raise Fog::AWS::IAM::NotFound.new("The group with name #{group_name} cannot be found.")
          end

          group = self.data[:groups][group_name]
          group[:attached_policies].delete(policy_arn)
          managed_policy["AttachmentCount"] -= 1

          Excon::Response.new.tap { |response|
            response.status = 200
            response.body = { "RequestId" => Fog::AWS::Mock.request_id  }
          }
        end