Add expression caching

app/services/billable_metrics/update_service.rb

@@ -40,6 +40,9 @@ def call
         billable_metric.recurring = params[:recurring] if params.key?(:recurring)
         billable_metric.weighted_interval = params[:weighted_interval]&.to_sym if params.key?(:weighted_interval)
         billable_metric.expression = params[:expression] if params.key?(:expression)
+        if params.key?(:expression) || params.key?(:field_name)
+          Rails.cache.delete("expression/#{organization.id}/#{billable_metric.code}")
+        end
       end
 
       billable_metric.save!

app/services/events/create_service.rb

@@ -42,13 +42,16 @@ def call
     attr_reader :organization, :params, :timestamp, :metadata
 
     def pre_ingest(event)
-      bm = organization.billable_metrics.with_expression.find_by(code: event.code)
-      return unless bm
+      field_name, expression = Rails.cache.fetch("expression/#{organization.id}/#{event.code}") do
+        bm = organization.billable_metrics.with_expression.find_by(code: event.code)
+        [bm&.field_name, bm&.expression]
+      end
+      return if expression.blank?
 
       evaluation_event = Lago::Event.new(event.code, event.timestamp.to_i, event.properties)
 
-      value = Lago::ExpressionParser.parse(bm.expression)&.evaluate(evaluation_event)
-      event.properties[bm.field_name] = value
+      value = Lago::ExpressionParser.parse(expression).evaluate(evaluation_event)
+      event.properties[field_name] = value
     end
 
     def produce_kafka_event(event)