Q:

Rails scope works different in spec and console

I have this class
class Invoice < ActiveRecord::Base
    scope :last_with_number, -> (firm) {firm.invoices.where('number IS NOT NULL').order("number ASC").last}
end

When I run Invoice.last_with_number(Firm.first).number in the console,  I get this error
2.0.0p247 :001 > Invoice.last_with_number(Firm.first).number
  Firm Load (1.5ms)  SELECT "firms".* FROM "firms" ORDER BY "firms"."id" ASC LIMIT 1
  Invoice Load (3.1ms)  SELECT "invoices".* FROM "invoices" WHERE "invoices"."firm_id" = $1 AND (number IS NOT NULL) ORDER BY number DESC LIMIT 1  [["firm_id", 1]]
NoMethodError:   Invoice Load (0.7ms)  SELECT "invoices".* FROM "invoices"
undefined method `number' for #<ActiveRecord::Relation::ActiveRecord_Relation_Invoice:0x00000008793198>

But this spec passes 
 describe Invoice do
    let(:firm)     {FactoryGirl.create(:firm)}
    let(:invoice1)   {FactoryGirl.create(:invoice, customer:customer, firm:firm , number: 2)}
    let(:invoice2)   {FactoryGirl.create(:invoice, customer:customer, firm:firm , number:3)}

    it 'gets the last numberd invoice for spesific firm' do 
        invoice1
        invoice2
        Invoice.last_with_number(firm).number.should eq 3
    end
 end

I can solve this problem by making a Invoice class method like this
def self.the_really_last(firm)
   last_with_number(firm).last
end

And change the scope from
scope :last_with_number, -> (firm) {firm.invoices.where('number IS NOT NULL').order("number ASC").last}

to
scope :last_with_number, -> (firm) {firm.invoices.where('number IS NOT NULL').order("number ASC")}

This will work both places.
But, why is the scope behaving differently in the the spec and the console? 

A:

I found out what it does. It happends when all records have number = nil. If you put last at the end of the scope it returns a ActiveRecord::Relation with all records. I made this repo to illustrate 
This rails issue gets a good explanation
To avoid confusion never use last first or all in scopes

