How can I make named_scope in Rails return one value instead of an array?

Question

I want to write a named scope to get a record from its id.

For example, I have a model called Event, and I want to simulate Event.find(id) with use of named_scope for future flexibility.

I used this code in my model:

named_scope :from_id, lambda { |id| {:conditions => ['id= ?', id] } }

and I call it from my controller like Event.from_id(id). But my problem is that it returns an array of Event objects instead of just one object.

Thus if I want to get event name, I have to write

event = Event.from_id(id)
event[0].name

while what I want is

event = Event.from_id(id)
event.name

Am I doing something wrong here?

Joshua Pinter · Accepted Answer · 2014-06-11 00:52:45Z

13

In this case I'd create a class method

def self.from_id(id)
  self.find(id).first
end

Another example:

class Invoice
  def self.number
    self.find(:first, 
      :select => "max(number) As last, max(number)+1 As next", 
      :conditions => "type = 'SalesOrder'")
  end
end

Methods like this its better then named_scope. You can call Invoice.number.next, better than named_scope return Invoice.number[0].next.

edited Jun 11, 2014 at 0:52

Joshua Pinter

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answered Mar 9, 2012 at 18:58

Gianpaulo

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Comments

Jimmy · Accepted Answer · 2010-05-29 07:18:15Z

10

As Damien mentions, using a custom scope just to look up by ID isn't advisable. But to answer your question:

Finding records with named_scope will always return an ActiveRecord::NamedScope::Scope object, which behaves like an array. If there is just a single record returned by your query, you can use the first method to get the ActiveRecord object directly, like this:

event = Event.from_id(id).first
event.name # will work as you expect

answered May 29, 2010 at 7:18

Jimmy

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1 Comment

sameera207 Over a year ago

hi Jimmy Thanks for the quick answer. Why I want to have a named scope is (as I mentioned in Damien's comment) because later I might want to get only the active events So then I can change only the named scope with out breaking the existing code (like only adding active=1) Please let me know your comments cheers, sameera

Damien MATHIEU · Accepted Answer · 2010-05-29 07:16:23Z

5

There's no need to use a scope here. You could just do Event.find_by_id id.
The difference between find and find_by_id is that the first one will raise an ActiveRecordNotFoundException if the record does not exist. The second one will just return nil.

Writing a scope to get the record by it's id is a very bad id as it is something which is already provided natively by rails.

answered May 29, 2010 at 7:16

Damien MATHIEU

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2 Comments

sameera207 Over a year ago

Hi Damien, thanks for the quick reply. Why i want to have this as a named scope is because later I might want to get only the active events So then I can change only the named scope with out breaking the existing code (like only adding active=1) Please let me know your comments cheers, sameera

P Shved Over a year ago

@sameera you could just add a generic method (def ... end); it doesn't have to be a named scope.

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