feat(unique-count): Add prorated unique count method

app/services/events/stores/postgres/unique_count_query.rb

@@ -34,6 +34,35 @@ def query
           SQL
         end
 
+        def prorated_query
+          <<-SQL
+            #{events_cte_sql},
+            event_values AS (
+              SELECT
+                property,
+                operation_type,
+                timestamp
+              FROM (
+                SELECT
+                  timestamp,
+                  property,
+                  operation_type,
+                  #{operation_value_sql} AS adjusted_value
+                FROM events_data
+                ORDER BY timestamp ASC
+              ) adjusted_event_values
+              WHERE adjusted_value != 0 -- adjusted_value = 0 does not impact the total
+              GROUP BY property, operation_type, timestamp
+            )
+
+            SELECT SUM(period_ratio) as aggregation
+            FROM (
+              SELECT (#{period_ratio_sql}) AS period_ratio
+              FROM event_values
+            ) cumulated_ratios
+          SQL
+        end
+
         # NOTE: Not used in production, only for debug purpose to check the computed values before aggregation
         # Returns an array of event's timestamp, property, operation type and operation value
         # Example:
@@ -62,7 +91,7 @@ def breakdown_query
 
         attr_reader :store
 
-        delegate :events, :sanitized_property_name, to: :store
+        delegate :events, :charges_duration, :sanitized_property_name, to: :store
 
         def events_cte_sql
           # NOTE: Common table expression returning event's timestamp, property name and operation type.
@@ -99,6 +128,21 @@ def operation_value_sql
             END
           SQL
         end
+
+        def period_ratio_sql
+          <<-SQL
+            CASE WHEN operation_type = 'add'
+            THEN
+              -- NOTE: duration in seconds between current event and next one - using end of period as final boundaries
+              EXTRACT(EPOCH FROM LEAD(timestamp, 1, :to_datetime) OVER (PARTITION BY property ORDER BY timestamp) - timestamp)
+              /
+              -- NOTE: full duration of the period
+              #{charges_duration.days.to_i}
+            ELSE
+              0 -- NOTE: duration was null so usage is null
+            END
+          SQL
+        end
       end
     end
   end

app/services/events/stores/postgres_store.rb

@@ -87,6 +87,21 @@ def unique_count_breakdown
         ).rows
       end
 
+      def prorated_unique_count
+        query = Events::Stores::Postgres::UniqueCountQuery.new(store: self)
+        sql = ActiveRecord::Base.sanitize_sql_for_conditions(
+          [
+            query.prorated_query,
+            {
+              to_datetime: to_datetime.ceil,
+            },
+          ],
+        )
+        result = ActiveRecord::Base.connection.select_one(sql)
+
+        result['aggregation']
+      end
+
       def max
         events.maximum("(#{sanitized_property_name})::numeric")
       end

spec/services/events/stores/postgres_store_spec.rb

@@ -160,6 +160,38 @@
     end
   end
 
+  describe '#prorated_unique_count' do
+    it 'returns the number of unique active event properties' do
+      create(
+        :event,
+        organization_id: organization.id,
+        external_subscription_id: subscription.external_id,
+        external_customer_id: customer.external_id,
+        code:,
+        timestamp: boundaries[:from_datetime] + 1.day,
+        properties: { billable_metric.field_name => 2 },
+      )
+
+      create(
+        :event,
+        organization_id: organization.id,
+        external_subscription_id: subscription.external_id,
+        external_customer_id: customer.external_id,
+        code:,
+        timestamp: boundaries[:from_datetime] + 2.days,
+        properties: {
+          billable_metric.field_name => 2,
+          operation_type: 'remove',
+        },
+      )
+
+      event_store.aggregation_property = billable_metric.field_name
+
+      # NOTE: Events calculation: 16/31 + 1/31 + 14/31 + 13/31 + 12/31
+      expect(event_store.prorated_unique_count.round(3)).to eq(1.806)
+    end
+  end
+
   describe '#events_values' do
     it 'returns the value attached to each event' do
       event_store.aggregation_property = billable_metric.field_name