Question

My Discount model has one Period association. I am trying to write a scope that select discounts that starts today, this includes:

1. when it has a period, select those periods that begins today.
2. when it does not have a period, select those discounts which were created between yesterday and today

My current query can do the first requirement (actually a bit complex than this):

def self.begins_today
  joins(:event).where("begin = ?", today)
end

However how can I achieve requirement 2?

I was thinking of using SQL UNION command, but I think it can’t work as a scope.

Answer 1

I’m assuming event contains the period association?

In any case you want a left join between the discounts table and the periods table. This will give you the period data to do the begin = today where clause, and null if there is no period. Thus the SQL to select the data would be

SELECT [columns]
FROM discounts_table
LEFT JOIN periods_table ON periods_table.discount_id = discounts_table.id
WHERE (periods_table.begin = [today]) OR (periods_table.begin IS NULL AND discounts_table.created_at BETWEEN [yesterday] AND [today])

in rails you should be able to achieve this as follows:

Discount
  .joins("LEFT JOIN periods_table ON periods_table.discount_id = discounts_table.id")
  .where("(periods_table.begin = ?) OR (periods_table.begin IS NULL AND discounts_table.created_at BETWEEN ? AND ?)", today, today, 1.day.ago.to_date)

Unfortunately you need the use SQL statements rather than letting rails create it for you as:

1. joins with a symbol only creates an INNER JOIN, not a LEFT JOIN
2. where with symbols, hashes etc will combine conditions using AND, not OR