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Wes Gamble

2006-Sep-18 23:49 UTC

Why doesn''t AR find method cause a call to obj.initialize?

All,

I have a method that does a find on an object by id, like so:

  contact = Contact.find(3)

A contact has a contact_preference, like so:

  has_one :contact_preference, :foreign_key => :ContactNumber, 
:dependent => true

In my Contact object, I''ve created an initialize method:

  def initialize(attributes = nil)
    super(attributes)
    self.create_contact_preference unless self.contact_preference
  end

The purpose of this initialize method is to add a contact_preference 
member to the contact if they don''t have one already in the database 
(which none of the legacy records do).

However, it is clear that my initialize method is not being called due 
to the find() invocation.

Does find() call a different constructor on an AR object?  If so, which 
one?

Am I creating my contact_preference member incorrectly?

Thanks,
Wes

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Wes Gamble

2006-Sep-19 00:29 UTC

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Re: Why doesn''t AR find method cause a call to obj.initializ

Well, I think I''ve verified the behavior that I''m seeing based
on the
code below and the use of allocate, instead of new.   As we can see, 
find() does not call initialize at all.

What I was hoping to be able to do here was to have the Contact object 
be smart enough to generate it''s contact_preference member when 
necessary.  I don''t think that this is an unreasonable expectation for 
an object.

I guess I''ve been thinking of Ruby''s new method as the
equivalent to new
in Java, but in fact, Ruby''s allocate is equivalent to Java''s
new.
This is slightly confusing.

Does anyone know why AR::Base uses allocate instead of new?  I''m sure 
that there''s a good reason.

I see that there is an "after_find()" callback available to me.  Is
this
what I need?

Thanks,
Wes

================================================================
def instantiate(record)
  object     if subclass_name = record[inheritance_column]
      if subclass_name.empty?
        allocate
      else
        require_association_class(subclass_name)
        begin
          compute_type(subclass_name).allocate
        rescue NameError
          raise SubclassNotFound,
            "The single-table inheritance mechanism failed to locate the 
subclass: ''#{record[inheritance_column]}''. " +
            "This error is raised because the column 
''#{inheritance_column}'' is reserved for storing the class in
case of
inheritance. " +
            "Please rename this column if you didn''t intend it to
be
used for storing the inheritance class " +
            "or overwrite #{self.to_s}.inheritance_column to use another 
column for that information."
        end
      end
    else
      allocate
    end

  object.instance_variable_set("@attributes", record)
  object
end


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Jonathan Viney

2006-Sep-19 00:43 UTC

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Re: Why doesn''t AR find method cause a call to obj.initialize?

initialize is called with Contact.new, Contact.find constructs object using
Contact.instantiate. You can probably do this:

class Contact
  class << self
    def instantiate_with_contact_preference(record)
      object = instantiate_without_contact_preference(record)
      # Do whatever you want with the object here
      object
    end

    alias_method_chain :instantiate, :contact_preference
  end
end

-Jonathan

On 9/19/06, Wes Gamble
<rails-mailing-list-ARtvInVfO7ksV2N9l4h3zg@public.gmane.org>
wrote:>
>
> All,
>
> I have a method that does a find on an object by id, like so:
>
>   contact = Contact.find(3)
>
> A contact has a contact_preference, like so:
>
>   has_one :contact_preference, :foreign_key => :ContactNumber,
> :dependent => true
>
> In my Contact object, I''ve created an initialize method:
>
>   def initialize(attributes = nil)
>     super(attributes)
>     self.create_contact_preference unless self.contact_preference
>   end
>
> The purpose of this initialize method is to add a contact_preference
> member to the contact if they don''t have one already in the
database
> (which none of the legacy records do).
>
> However, it is clear that my initialize method is not being called due
> to the find() invocation.
>
> Does find() call a different constructor on an AR object?  If so, which
> one?
>
> Am I creating my contact_preference member incorrectly?
>
> Thanks,
> Wes
>
> --
> Posted via http://www.ruby-forum.com/.
>
> >
>

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Wes Gamble

2006-Sep-19 00:51 UTC

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Re: Why doesn''t AR find method cause a call to obj.initializ

after_find is what I needed.

I''m still kind of annoyed at the difference in behavior.

I''m trying to figure out why finders
couldn''t/shouldn''t call initialize
methods.

What''s the difference in me applying my object customizations to my 
object in after_find vs. initialize?

If the finders called initialize, then the instantiation model for 
AR::Base descendants would be consistent.

As it stands now, I have AR::Base descendants that do "create from 
scratch" initialization in the initialize method and other AR::Base 
descendants that do "create from persisted DB data" initialization in 
the "after_find" method.

For someone new to Ruby as I am, who is already comfortable with OO, 
it''s weird to have to do object initialization a special way just for 
retrieved DB objects.

Wes

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Wes Gamble

2006-Sep-19 00:55 UTC

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Re: Why doesn''t AR find method cause a call to obj.initializ

Jonathan,

This is a valid solution - thanks for it.

However, it begs the question - why the difference in object 
instantiation?

Wes


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Josh Susser

2006-Sep-19 07:09 UTC

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Re: Why doesn''t AR find method cause a call to obj.initializ

Just a guess here.  When you find() a record, you aren''t creating a 
brand new object, but are reconstructing an old object from a persistent 
representation of it stored in the database. So AR::initialize isn''t 
useful since it can''t tell whether it would be for a new object or a 
reconstructed one. Therefore you use after_initialize or after_find to 
init the required attributes.

Or maybe it''s something else.  Like I said, I''m just guessing.

--
Josh Susser
http://blog.hasmanythrough.com

Wes Gamble wrote:> This is a valid solution - thanks for it.
> 
> However, it begs the question - why the difference in object 
> instantiation?

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Wes Gamble

2006-Sep-19 16:03 UTC

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Re: Why doesn''t AR find method cause a call to obj.initializ

Josh Susser wrote:> Just a guess here.  When you find() a record, you aren''t creating
a
> brand new object, but are reconstructing an old object from a persistent 
> representation of it stored in the database. So AR::initialize
isn''t
> useful since it can''t tell whether it would be for a new object or
a
> reconstructed one. Therefore you use after_initialize or after_find to 
> init the required attributes.
Yes, but in reality, I am creating a new object in memory regardless of 
whether I do it directly with new() or indirectly with find().  I should 
be able to have an initialization hook that is consistent regardless of 
instantiation method.  If I''m doing stuff to my AR::Base descendant
that
doesn''t even have anything to do with the database, than why
can''t that
work be done the same way regardless of how the object is created?

Clearly the hooks are there, I''m just wondering why/complaining that 
they aren''t the same.  Just another detail that I have to master, and I
wanna know why :).

Wes

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Rails - Sep 2006 - Why doesn't AR find method cause a call to obj.initialize?

Why doesn''t AR find method cause a call to obj.initialize?

Re: Why doesn''t AR find method cause a call to obj.initializ

Re: Why doesn''t AR find method cause a call to obj.initialize?

Re: Why doesn''t AR find method cause a call to obj.initializ

Re: Why doesn''t AR find method cause a call to obj.initializ

Re: Why doesn''t AR find method cause a call to obj.initializ

Re: Why doesn''t AR find method cause a call to obj.initializ