Add helper for grouping by periods

The methods are still experimental and might change without notice!

You need to add `include DateGroupable` to your model before you can use it like this:
`User.smart_group_by_date("users.created_at", start_date, end_date)`.count

app/models/concerns/date_groupable.rb

@@ -0,0 +1,48 @@
+module DateGroupable extend ActiveSupport::Concern
+  class_methods do
+    def group_by_day(column)
+      group_by_unit(:day, column)
+    end
+
+    def group_by_week(column)
+      group_by_unit(:week, column)
+    end
+
+    def group_by_month(column)
+      group_by_unit(:month, column)
+    end
+
+    def group_by_quarter(column)
+      group_by_unit(:quarter, column)
+    end
+
+    def group_by_year(column)
+      group_by_unit(:year, column)
+    end
+
+    def group_by_unit(aggregation_unit, column)
+      group("date_trunc('#{aggregation_unit}', #{column})")
+        .order("date_trunc('#{aggregation_unit}', #{column})")
+    end
+
+    def smart_group_by_date(column, start_date, end_date)
+      days = (start_date.to_date..end_date.to_date).count
+
+      case
+      when days <= 40
+        aggregation_unit = :day
+      when days <= 210  # 30 weeks
+        aggregation_unit = :week
+      when days <= 550  # ~18 months
+        aggregation_unit = :month
+      when days <= 1461  # ~4 years
+        aggregation_unit = :quarter
+      else
+        aggregation_unit = :year
+      end
+
+      where("#{column} BETWEEN ? AND ?", start_date, end_date)
+        .group_by_unit(aggregation_unit, column)
+    end
+  end
+end