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Ben Hellerstein

2007-May-17 20:12 UTC

Model: self is not child''s parent

Assume a model Parent and another model Child.  Child belongs_to Parent
and Parent has_many Children.  The following fails:

@
p = Parent.find(:first)
assert p.equal?(p.children[0].parent)
@

It seems odd to me that I and my child''s parent are not the same
object.
In fact, this is royally screwing me up.  Can anyone tell me why this is
or point me at a relevant discussion, blog posting, or bug that refers
to this.

Thanks!
-Ben

-- 
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Ben Hellerstein

2007-May-17 20:13 UTC

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Re: Model: self is not child''s parent

Ignore the spurious at-signs

-- 
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Pete Yandell

2007-May-18 00:05 UTC

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Re: Model: self is not child''s parent

On 18/05/2007, at 6:12 AM, Ben Hellerstein wrote:
> Assume a model Parent and another model Child.  Child belongs_to  
> Parent
> and Parent has_many Children.  The following fails:
>
> @
> p = Parent.find(:first)
> assert p.equal?(p.children[0].parent)
> @
>
> It seems odd to me that I and my child''s parent are not the same  
> object.
> In fact, this is royally screwing me up.  Can anyone tell me why  
> this is
> or point me at a relevant discussion, blog posting, or bug that refers
> to this.
ActiveRecord (at least in Rails 1.2.3) doesn''t do object caching, so  
following the relationships down and back up again will give you a  
new object. However:

     assert p == p.children[0].parent

should still succeed.

Pete Yandell
http://notahat.com/

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Amos King

2007-May-18 13:25 UTC

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Re: Model: self is not child''s parent

a.equal?(b) iff a is the same object as b

The eql? method returns true if obj and anObject have the same value

On 5/17/07, Pete Yandell <pete-AylieETRJAdBDgjK7y7TUQ@public.gmane.org>
wrote:>
>
> On 18/05/2007, at 6:12 AM, Ben Hellerstein wrote:
>
> > Assume a model Parent and another model Child.  Child belongs_to
> > Parent
> > and Parent has_many Children.  The following fails:
> >
> > @
> > p = Parent.find(:first)
> > assert p.equal?(p.children[0].parent)
> > @
> >
> > It seems odd to me that I and my child''s parent are not the
same
> > object.
> > In fact, this is royally screwing me up.  Can anyone tell me why
> > this is
> > or point me at a relevant discussion, blog posting, or bug that refers
> > to this.
>
> ActiveRecord (at least in Rails 1.2.3) doesn''t do object caching,
so
> following the relationships down and back up again will give you a
> new object. However:
>
>      assert p == p.children[0].parent
>
> should still succeed.
>
> Pete Yandell
> http://notahat.com/
>
> >
>

-- 
Amos King
Ramped Media
USPS
Programmer/Analyst
St. Louis, MO
Looking for something to do? Visit ImThere.com

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Robert Walker

2007-May-18 21:26 UTC

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Re: Model: self is not child''s parent

Seems to work perfectly for me.

parent.rb
class Parent < ActiveRecord::Base
  has_many :children
end

child.rb
class Child < ActiveRecord::Base
  belongs_to :parent
end

parent_test.rb
  def test_child_parent_is_parent
    parent = Parent.find(:first)
    assert_equal parent, parent.children[0].parent
  end

003_load_test_data.rb
class LoadTestData < ActiveRecord::Migration
  def self.up
    parent = Parent.create(:name => "Robert")
    Child.create(:name => "Bill", :parent => parent)
    Child.create(:name => "Sally", :parent => parent)
  end

  def self.down
    Child.delete_all
    Parent.delete_all
  end
end

Test results:
Rigel$ ruby test/unit/parent_test.rb
Loaded suite test/unit/parent_test
Started
.
Finished in 0.10244 seconds.

1 tests, 1 assertions, 0 failures, 0 errors

On May 17, 4:13 pm, Ben Hellerstein
<rails-mailing-l...-ARtvInVfO7ksV2N9l4h3zg@public.gmane.org>
wrote:> Ignore the spurious at-signs
>
> --
> Posted viahttp://www.ruby-forum.com/.

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Robert Walker

2007-May-18 21:38 UTC

head link

Re: Model: self is not child''s parent

But you are correct that this is two separate instances of the Parent
object.  Rails does not seem to have a uniquing system so you''ll need
to compare object equality using methods supplied by ActiveRecord,
which is how "assert_equal" is going to determine equality.

Basically, you have two separate instances of Parent representing the
same row in the parents table.  Building a system that ensures object
equality in object relational mapping is a pretty complex endeavor.
It appears that Rails makes no attempt to do this.

On May 18, 5:26 pm, Robert Walker
<rwalker...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:> Seems to work perfectly for me.
>
> parent.rb
> class Parent < ActiveRecord::Base
>   has_many :children
> end
>
> child.rb
> class Child < ActiveRecord::Base
>   belongs_to :parent
> end
>
> parent_test.rb
>   def test_child_parent_is_parent
>     parent = Parent.find(:first)
>     assert_equal parent, parent.children[0].parent
>   end
>
> 003_load_test_data.rb
> class LoadTestData < ActiveRecord::Migration
>   def self.up
>     parent = Parent.create(:name => "Robert")
>     Child.create(:name => "Bill", :parent => parent)
>     Child.create(:name => "Sally", :parent => parent)
>   end
>
>   def self.down
>     Child.delete_all
>     Parent.delete_all
>   end
> end
>
> Test results:
> Rigel$ ruby test/unit/parent_test.rb
> Loaded suite test/unit/parent_test
> Started
> .
> Finished in 0.10244 seconds.
>
> 1 tests, 1 assertions, 0 failures, 0 errors
>
> On May 17, 4:13 pm, Ben Hellerstein
<rails-mailing-l...-ARtvInVfO7ksV2N9l4h3zg@public.gmane.org>
> wrote:
>
> > Ignore the spurious at-signs
>
> > --
> > Posted viahttp://www.ruby-forum.com/.

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Robert Walker

2007-May-18 22:06 UTC

head link

Re: Model: self is not child''s parent

A bit more detail too offer.....
>From the ri on equal?:----------------------------------------------------------
Object#equal?
     obj == other        => true or false
     obj.equal?(other)   => true or false
     obj.eql?(other)     => true or false
------------------------------------------------------------------------
     Equality---At the +Object+ level, +==+ returns +true+ only if
_obj_
     and _other_ are the same object. Typically, this method is
     overridden in descendent classes to provide class-specific
meaning.

     Unlike +==+, the +equal?+ method should never be overridden by
     subclasses: it is used to determine object identity (that is,
     +a.equal?(b)+ iff +a+ is the same object as +b+).

     The +eql?+ method returns +true+ if _obj_ and _anObject_ have the
     same value. Used by +Hash+ to test members for equality. For
     objects of class +Object+, +eql?+ is synonymous with +==+.
     Subclasses normally continue this tradition, but there are
     exceptions. +Numeric+ types, for example, perform type conversion
     across +==+, but not across +eql?+, so:

        1 == 1.0     #=> true
        1.eql? 1.0   #=> false
-------------------

And both these assertions pass just fine:
  def test_child_parent_is_parent
    parent = Parent.find(:first)
    assert_equal(parent, parent.children[0].parent)
    assert(parent.eql?(parent.children[0].parent))
  end


On May 18, 5:38 pm, Robert Walker
<rwalker...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:> But you are correct that this is two separate instances of the Parent
> object.  Rails does not seem to have a uniquing system so you''ll
need
> to compare object equality using methods supplied by ActiveRecord,
> which is how "assert_equal" is going to determine equality.
>
> Basically, you have two separate instances of Parent representing the
> same row in the parents table.  Building a system that ensures object
> equality in object relational mapping is a pretty complex endeavor.
> It appears that Rails makes no attempt to do this.
>
> On May 18, 5:26 pm, Robert Walker
<rwalker...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
>
> > Seems to work perfectly for me.
>
> > parent.rb
> > class Parent < ActiveRecord::Base
> >   has_many :children
> > end
>
> > child.rb
> > class Child < ActiveRecord::Base
> >   belongs_to :parent
> > end
>
> > parent_test.rb
> >   def test_child_parent_is_parent
> >     parent = Parent.find(:first)
> >     assert_equal parent, parent.children[0].parent
> >   end
>
> > 003_load_test_data.rb
> > class LoadTestData < ActiveRecord::Migration
> >   def self.up
> >     parent = Parent.create(:name => "Robert")
> >     Child.create(:name => "Bill", :parent => parent)
> >     Child.create(:name => "Sally", :parent => parent)
> >   end
>
> >   def self.down
> >     Child.delete_all
> >     Parent.delete_all
> >   end
> > end
>
> > Test results:
> > Rigel$ ruby test/unit/parent_test.rb
> > Loaded suite test/unit/parent_test
> > Started
> > .
> > Finished in 0.10244 seconds.
>
> > 1 tests, 1 assertions, 0 failures, 0 errors
>
> > On May 17, 4:13 pm, Ben Hellerstein
<rails-mailing-l...-ARtvInVfO7ksV2N9l4h3zg@public.gmane.org>
> > wrote:
>
> > > Ignore the spurious at-signs
>
> > > --
> > > Posted viahttp://www.ruby-forum.com/.

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Ben Hellerstein

2007-May-18 23:13 UTC

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Re: Model: self is not child''s parent

Thanks for answering my question.  I suspected the complexity involved 
in ensuring that self and child''s parent are not just the same database
row but also the same instance was prohibitive.  Looks like I was right.

-Ben

-- 
Posted via http://www.ruby-forum.com/.

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jimcfisher-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org

2007-May-22 17:38 UTC

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Re: Model: self is not child''s parent

you want.

assert p.eql? not p.equal? (equal is more restrictive/exact than
eql?).

I''m not entirely sure, but I believe when you call
p.children[0].parent you are actually instantiating in memory an new
object of Parent class. So,  #<Parent:0x39f3018 > is exactly equal to
#<Parent:0x39f3020 >

so in the end use == or eql? instead of equal? if your not wanting to
check for EXACTLY the same instance of Parent.

Hope that helps,
Jim

http://www.wellho.net/mouth/985_Equality-in-Ruby-eql-and-equal-.html

On May 17, 3:13 pm, Ben Hellerstein
<rails-mailing-l...-ARtvInVfO7ksV2N9l4h3zg@public.gmane.org>
wrote:> Ignore the spurious at-signs
>
> --
> Posted viahttp://www.ruby-forum.com/.

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