rails scope cannot generate correct sql

Question

I added a variable in config/application.rb:

config.available_account_types = %w(SystemAccount CashAccount DemandAccount LiabilityAccount CreditCardAccount)

And generated some scopes in model account.rb:

for t in Mypurse::Application.config.available_account_types
  scope t.underscore.pluralize.to_sym, -> {where(type: t)}
end

But when I try all of them, Account.system_accounts, Account.cash_accounts, etc, I got this sql for every account type:

where type = 'CreditCardAccount'

That is, all of the generated scope are pointed to the {where(type: 'CreditCardACcount')}

I don't know why.

here is the source file: https://github.com/chylli/mypurse/blob/add_cash_demand_liability_credit_card/config/application.rb

https://github.com/chylli/mypurse/blob/add_cash_demand_liability_credit_card/app/models/account.rb

nathanvda · Accepted Answer · 2014-07-01 08:59:26Z

1

I think this is caused because a scope is given a Proc which is only executed when called, and so t will always be the last element of the loop.

A solution is to define methods instead of scopes (which work exactly the same) :

MyPurs::Application.config.available_account_types.each do |account_type| 
  define_singleton_method(account_type.underscore.pluralize.to_sym) do 
    where(type: "#{account_type}")
  end
end

But since this does not declare a proc, this should work as expected. Also the for .. in is rarely used in ruby, I personally prefer to use the more idiomatic .each (but of course you are free to use whatever you want, programmer happiness is key in ruby :) :)

Now as an aside, while meta-programming is really cool, you should really ask yourself if just listing the scopes is not way more readable. I understand: meta-programming is more DRY, but personally, in most cases where I did this, I reverted to the explicit definitions because of readability.

answered Jul 1, 2014 at 8:59

nathanvda

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5 Comments

chylli Over a year ago

Thanks for your answer. Is there no way to create a new local variable for every running of the loop ? I'm a PERL programmer so I always think the problem with the PERL's behavior.

nathanvda Over a year ago

So the way I showed you should work. Defining scopes would not, because it defines a Proc (the ->) which delays execution until it is called.

chylli Over a year ago

I changed my version to this and worked: proc {|t2| scope t2.underscore.pluralize.to_sym,-> {where(type:t2)}}.call(t)

nathanvda Over a year ago

If that is your preferred solution, you should enter it as an answer and accept it instead. Personally I find my solution much more readable, and it behaves exactly the same.

chylli Over a year ago

I agree with you. I just want to study ruby deeply. I think your answer is better than mine.

CuriousMind · Accepted Answer · 2014-07-01 09:14:25Z

0

I am not sure why you have defined 'config.available_account_types', as this is business logic. this should belong to Account modal. so I would do something like this

class Account < ActiveRecord::Base
  ACCOUNT_TYPES = %w(SystemAccount CashAccount DemandAccount LiabilityAccount CreditCardAccount)
  ACCOUNT_TYPES.each do |acccount_type|
    define_singleton_method(account_type.underscore.pluralize.to_sym) do 
    where(type: "#{account_type}")
  end
end
end

edited Jul 1, 2014 at 9:14

answered Jul 1, 2014 at 9:02

CuriousMind

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2 Comments

nathanvda Over a year ago

Looks strangely familiar. But good remark about the account types. You are missing an end though.

chylli Over a year ago

the reason I define this variable is that I will use it in another module. I tried define it as a function types in the Account module but the User module will say 'missing method'. OK I will remove the meta-program. thanks very much.

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