Given a timestamp, how to get 1 month ago - ruby-on-rails

Question

I'm trying to build a method where I pass a month and can then query for the previous month dynamically.
total_churn(month)
last_month = month - 1
companies = Company.where("created_at BETWEEN '#{last_month}' AND '#{month}')
return companies.count
end
How do I pass the method 'month' in a way where I can dynamically determine the last month using ruby on rails? Thanks

Answer 1

My suggestion: accept a date rather than a month.
total_churn(date)
month_previous = date - 1.month
companies = Company.where("created_at BETWEEN ? AND ?, '#{month_previous}', '#{date}')
return companies.count
end
Current month:
Time.now.month
Date.today.month
Time or day one month ago:
(Time.now - 1.month).month
(Date.today - 1.month).month
...also equivalent to:
Time.now.month - 1
Date.today.month - 1
Previous month for any given date:
#date - 1.month
I would personally build your method to accept a date rather than just a month number. As long as the created_at field is storing dates, you'll need to give the query two dates for it to run, even if those dates are the 1st.

Answer 2

Rails has some helpful time helpers to create upper and lower bounds for your query. (beginning_of_month and end_of_month in the Time class)
This method is also properly escaped with question marks instead of string interpolation, which is open to SQL injection attacks.
def total_churn(month)
companies = Company.where('created_at BETWEEN ? and ?',(Time.now - 1.month).beginning_of_month,(Time.now - 1.month).end_of_month)
companies.count
end
I would also say that this will only work for the most recent year. If you want to be able to query for earlier data, you may want to either add a year param or simply pass in a date and let it use that in place of Time.now.
# with separate year and month params
def total_churn(month, year)
date = DateTime.parse("#{year}/#{month}/01")
companies = Company.where('created_at BETWEEN ? and ?',(date - 1.month).beginning_of_month,(date - 1.month).end_of_month)
companies.count
end
# with a random date input
def total_churn(date_in_month)
companies = Company.where('created_at BETWEEN ? and ?',(date_in_month - 1.month).beginning_of_month,(date_in_month - 1.month).end_of_month)
companies.count
end

Answer 3

You need at least 2 parameters to accomplish this, one for the month and one for the year. This is needed to establish what month you want.
def total_churn(year, month)
date = Date.new year, month
one_month_ago = date - 1.month
...
However, if you already have a date object, then you can use sscirrus' answer. If the date is in a string, you'd probably want to parse it first (like in your comment)
def total_churn(date_string)
date = Date.parse(date_string)
one_month_ago = date - 1.month
...
>> date_string = '2012-09-01 00:00:00.000000'
>> total_churn date_string