Issue warning when the error is a precision error

lib/rspec/rails/matchers/active_job.rb

@@ -163,7 +163,27 @@ def at_match?(job)
             return job[:at].nil? if @at == :no_wait
             return false unless job[:at]
 
-            values_match?(@at, Time.at(job[:at]))
+            scheduled_at = Time.at(job[:at])
+            values_match?(@at, scheduled_at) || check_for_inprecise_value(scheduled_at)
+          end
+
+          def check_for_inprecise_value(scheduled_at)
+            return unless Time === @at && values_match?(@at.change(usec: 0), scheduled_at)
+
+            RSpec.warn_with((<<-WARNING).gsub(/^\s+\|/, '').chomp)
+            |[WARNING] Your expected `at(...)` value does not match the job scheduled_at value
+            |unless microseconds are removed. This precision error often occurs when checking
+            |values against `Time.current` / `Time.now` which have usec precision, but Rails
+            |uses `n.seconds.from_now` internally which has a usec count of `0`.
+            |
+            |Use `change(usec: 0)` to correct these values.
+            |
+            |Note: RSpec cannot do this for you because jobs can be scheduled with usec
+            |precision and we do not know wether it is on purpose or not.
+            |
+            |
+            WARNING
+            false
           end
 
           def set_expected_number(relativity, count)

spec/rspec/rails/matchers/active_job_spec.rb

@@ -233,6 +233,17 @@ def self.name; "LoggingJob"; end
       end
     end
 
+    it "warns when time offsets are inprecise" do
+      expect(RSpec).to receive(:warn_with).with(/precision error/)
+
+      time = Time.current.change(usec: 550)
+      travel_to time do
+        expect {
+          expect { hello_job.set(wait: 5).perform_later }.to have_enqueued_job.at(time + 5)
+        }.to raise_error(/expected to enqueue exactly 1 jobs/)
+      end
+    end
+
     it "accepts composable matchers as an at date" do
       future = 1.minute.from_now
       slightly_earlier = 58.seconds.from_now