Give ChangeDetectorAgent access to last_property

This allows user to detect a new lowest value or a new high by
comparing a new value with the existing value.

app/models/agents/change_detector_agent.rb

@@ -5,7 +5,7 @@ module Agents
     description <<-MD
       The Change Detector Agent receives a stream of events and emits a new event when a property of the received event changes.
 
-      `property` specifies the property to be watched.
+      `property` specifies a Liquid template that expands to the property to be watched, where you can use a variable `last_property` for the last property value.  If you want to detect a new lowest price, try this: `{% assign drop = last_property | minus: price %}{% if last_property == blank or drop > 0 %}{{ price | default: last_property }}{% else %}{{ last_property }}{% endif %}`
 
       `expected_update_period_in_days` is used to determine if the Agent is working.
 
@@ -43,7 +43,10 @@ module Agents
 
     def receive(incoming_events)
       incoming_events.each do |event|
-        handle(interpolated(event), event)
+        interpolation_context.stack do
+          interpolation_context['last_property'] = last_property
+          handle(interpolated(event), event)
+        end
       end
     end
 

spec/models/agents/change_detector_agent_spec.rb

@@ -95,4 +95,62 @@ describe Agents::ChangeDetectorAgent do
       }.to change(Event, :count).by(0)
     end
   end
+
+  describe "#receive using last_property to track lowest value" do
+    before :each do
+      @event = create_event("100")
+    end
+
+    before do
+      # Evaluate the output as number and detect a new lowest value
+      @checker.options['property'] = '{% assign drop = last_property | minus: output %}{% if last_property == blank or drop > 0 %}{{ output | default: last_property }}{% else %}{{ last_property }}{% endif %}'
+    end
+
+    it "creates events when the value drops" do
+      @checker.receive([@event])
+
+      event = create_event("90")
+      expect {
+        @checker.receive([event])
+      }.to change(Event, :count).by(1)
+      expect(@checker.memory['last_property']).to eq "90"
+    end
+
+    it "does not create event when the value does not change" do
+      @checker.receive([@event])
+
+      event = create_event("100")
+      expect {
+        @checker.receive([event])
+      }.not_to change(Event, :count)
+      expect(@checker.memory['last_property']).to eq "100"
+    end
+
+    it "does not create event when the value rises" do
+      @checker.receive([@event])
+
+      event = create_event("110")
+      expect {
+        @checker.receive([event])
+      }.not_to change(Event, :count)
+      expect(@checker.memory['last_property']).to eq "100"
+    end
+
+    it "does not create event when the value is blank" do
+      @checker.receive([@event])
+
+      event = create_event("")
+      expect {
+        @checker.receive([event])
+      }.not_to change(Event, :count)
+      expect(@checker.memory['last_property']).to eq "100"
+    end
+
+    it "creates events when memory is empty" do
+      expect {
+        @checker.receive([@event])
+      }.to change(Event, :count).by(1)
+      expect(@checker.memory['last_property']).to eq "100"
+    end
+  end
 end