Limit the date range for date dimension ETL

If a user entered a date for an event that was way in the past or future
it would cause the ETL process to create a large number of records
unnecessarily.  Therefore, the date range is being limited in order to
prevent this problem from occuring.

app/analytics/etl/dimensions/date.rb

@@ -1,6 +1,6 @@
 class Etl::Dimensions::Date
-    def self.run
-        load_date_range
+    def self.run(zero_day: default_zero_day, future_window: default_future_window)
+        load_date_range zero_day, future_window
         load_null_date
     end
 
@@ -22,13 +22,21 @@ def self.day_number_in_school_year(date)
         (date - first_day_of_school(date)).to_i + 1
     end
 
+    def self.default_future_window
+      1.year.from_now.to_date
+    end
+
+    def self.default_zero_day
+        Date.new(2015, 1, 1)
+    end
+
     def self.first_day_of_school(date)
         (school_year_number(date).to_s + '-07-01').to_date
     end
 
-    def self.load_date_range
-        date = NetworkEvent.minimum(:scheduled_at).to_date
-        max_date = NetworkEvent.maximum(:scheduled_at).to_date
+    def self.load_date_range(zero_day, future_window)
+        date = minimum_date(zero_day)
+        max_date = maximum_date(future_window)
         while date <= max_date do
             attributes = {
                 date: date.to_date,
@@ -93,6 +101,14 @@ def self.load_null_date
         persist_date attributes
 
     end
+
+    def self.maximum_date(future_window)
+        [future_window, NetworkEvent.maximum(:scheduled_at).to_date].min
+    end
+
+    def self.minimum_date(zero_day)
+        [zero_day, NetworkEvent.minimum(:scheduled_at).to_date].max
+    end
 
     def self.persist_date(attributes)
         if DateDimension.where(date: attributes[:date]).exists?

test/etl/dimensions/date_test.rb

@@ -23,6 +23,28 @@ def setup
     assert_equal days, DateDimension.where.not(date: nil).count
   end
 
+  test 'No date is created before zero day' do
+    minimum_date = DateDimension.where.not(date: nil).order(:date).first
+    minimum_date.delete
+
+    zero_day = minimum_date.date + 1.day
+    Etl::Dimensions::Date.run zero_day: zero_day
+
+    new_minimum_date = DateDimension.where.not(date: nil).order(:date).first
+    assert_equal zero_day, new_minimum_date.date
+  end
+
+  test 'No date is created beyond the future window' do
+    maximum_date = DateDimension.where.not(date: nil).order(date: :desc).first
+    maximum_date.delete
+
+    future_window = maximum_date.date - 1.day
+    Etl::Dimensions::Date.run future_window: future_window
+
+    new_maximum_date = DateDimension.where.not(date: nil).order(date: :desc).first
+    assert_equal future_window, new_maximum_date.date
+  end
+
   test 'A date is created for a missing date' do
     assert_equal 1, DateDimension.where(date: nil).count
   end