Rails scope doesn't return correct data

Question

When I use scope which I prepared in model, rails returns incorrect data.

My Model:

class CurrencyRate < ActiveRecord::Base
    scope :eur_today, -> {where(currency: "eur").where(date: Time.now.in_time_zone.to_date).first}
end

Inforamation from rails console:

2.3.0 :011 > CurrencyRate.eur_today

CurrencyRate Load (0.2ms)  SELECT  "currency_rates".* FROM 
"currency_rates" WHERE "currency_rates"."currency" = ? AND 
"currency_rates"."date" = ?  ORDER BY "currency_rates"."id" ASC LIMIT 1  
[["currency", "eur"], ["date", "2017-08-09"]]
CurrencyRate Load (0.2ms)  SELECT "currency_rates".* FROM 
"currency_rates"

 => #<ActiveRecord::Relation [#<CurrencyRate id: 2, currency: "eur", 
 sale: 4.248546249999998, purchase: 4.265333125, date: "2017-08-08", 
 created_at: "2017-08-08 20:52:08", updated_at: "2017-08-08 20:54:10", 
 sale_percentage_diff: nil, purchase_percentage_diff: nil>]>

When I use the same query like in scope, returned data is correct:

2.3.0 :012 > CurrencyRate.where(currency: "eur").where(date: Time.now.in_time_zone.to_date).first 

CurrencyRate Load (0.2ms)  SELECT  "currency_rates".* FROM 
"currency_rates" WHERE "currency_rates"."currency" = ? AND 
"currency_rates"."date" = ?  ORDER BY "currency_rates"."id" ASC LIMIT 1  
[["currency", "eur"], ["date", "2017-08-09"]]

=> nil

Why scope doesn't work correct?

Your scope is not a real scope because it returns the first found CurrencyRate record. Remove that .first in your scope. You can see in your IRB that there are 2 generated SQL queries, one with the limit 1 (because of your .first) and another just doing "select all from current_rates" — MrYoshiji
– MrYoshiji, Commented Aug 9, 2017 at 20:42

Ursus · Accepted Answer · 2017-08-09 20:37:46Z

3

Because scopes must return an ActiveRecord::Relation. Get rid of the first call in your scope and use that outside the scope. Why? Because scopes have to be chainable.

answered Aug 9, 2017 at 20:37

Ursus

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3 Comments

engineersmnky Over a year ago

or just make it a class instance method e.g. def self.eur_today since the "scope" name seems to suggest the intention is to return the rate of the Euro as of today. Although I would probably make this def self.currency_value(currency,as_of=Time.now.in_time_zone.to_date) so that it can be reused better

Ursus Over a year ago

Yep, that's an alternative

Damian Over a year ago

I tried both ways. I understand now how scopes work. Thanks.

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