activerecord equivalent to SQL 'minus'

问题

What's the rails way to subtract a query result from another? A database specific SQL example would be:

SELECT Date FROM Store_Information
MINUS
SELECT Date FROM Internet_Sales 

回答1:

I'll throw this into the mix - not a solution, but might help with the progress:

Best I can think of is to use NOT IN:

StoreInformation.where('date NOT IN (?)', InternetSale.all)

That's Rails 3 - Rails 2 would be:

StoreInformation.all(:conditions => ['date NOT IN(?)', InternetSale.all])

But both of these will first select everything from internet_sales; what you really want is a nested query to do the whole thing in the database engine. For this, I think you'll have to break into find_by_sql and just give a nested query.

Obviously this assumes you're using MySQL! HTH.

回答2:

Late answer but I think you meant :

activities = Activity.all

searches = activites.where(key: 'search')

(activites - searches).each do |anything_but_search|
   p anything_but_search
end

You can substract two ActiveRecordsRelation and get the MINUS result, just like SQL would.

I am using Rails 4.2 so anything beyond that version should do the trick.

来源：https://stackoverflow.com/questions/3428852/activerecord-equivalent-to-sql-minus