Question

Before I start, I apologize for a bad title but I could not come up with one that describes my question in a satisfying way. If you come up with a better title, I will gladly switch.

Suppose I have an Account model and a Transaction model, and I would like to implement a Account#days_since_balance_was_atleast method, taking a sum as its sole parameter. What would be an effective way to do this?

Here comes some sample code:

class Transaction < ActiveRecord::Base
    validates_presence_of :account_id, :created_at, :amount
    belongs_to :account
    # Some logic to update account balance at creation...
end

class Account < ActiveRecord::Base
   validates_presence_of :balance 
   has_many :transactions

   def days_since_balance_was_atleast(sum)
       #How should I implement this?
   end
end

Should I use some smart SQL fragment, or perhaps ActiveRecord::Calculations? Loading all transactions and stepping backwards manually seems like a very bad idea (especially since there can be lots of transactions). And if I am stuck with the last approach, do you think it would be smart to retrieve the transactions in batches.

What it should produce (Updated):

# We have the following transactions 
# (time is unimportant for the result unless it is 00.00)
# Days ago  0   1   2   3   4  5   6   7
# Amount    -20 10 -60 -30 50 50 -100 100

account.balance == 0             # true
days_since_balance_was(100) == 3 # true

So, how would you solve this problem?

Answer 1

Shouldn’t be that hard, using SQL join. Something along the lines of:

SELECT t1.time
FROM transactions t1
LEFT JOIN transactions t2 ON t2.time < t1.time
HAVING SUM(t2.amount) >= 100
GROUP BY t1.id, t1.time
ORDER BY t1.time
LIMIT 1

Either implement it using AR, or create a view in your database.