# Model: self is not child's parent **URL:** \ **Category:** Rails\ **Created:** [May 17, 2007, 10:12pm UTC](https://www.ruby-forum.com/t/model-self-is-not-childs-parent/104436 "2007-05-17T22:12:08Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![benhiller](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/8e7dd6/32.png) [@benhiller](https://www.ruby-forum.com/u/benhiller)\ **Post date:** [May 17, 2007, 10:12pm UTC](https://www.ruby-forum.com/t/model-self-is-not-childs-parent/104436/1 "2007-05-17T22:12:08Z")
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 ---
**Author:** ![benhiller](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/8e7dd6/32.png) [@benhiller](https://www.ruby-forum.com/u/benhiller)\ **Post date:** [May 17, 2007, 10:13pm UTC](https://www.ruby-forum.com/t/model-self-is-not-childs-parent/104436/2 "2007-05-17T22:13:17Z")
Ignore the spurious at-signs ---
**Author:** ![benhiller](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/8e7dd6/32.png) [@benhiller](https://www.ruby-forum.com/u/benhiller)\ **Post date:** [May 18, 2007, 2:06am UTC](https://www.ruby-forum.com/t/model-self-is-not-childs-parent/104436/3 "2007-05-18T02:06:55Z")
On 18/05/2007, at 6:12 AM, Ben H. wrote: > 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 Y. > **[Pete Yandell — Principal Engineer / Architect](https://notahat.com/)** > > Pete Yandell is an experienced Principal Software Engineer and Architect based in Melbourne, Australia. ---
**Author:** ![benhiller](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/8e7dd6/32.png) [@benhiller](https://www.ruby-forum.com/u/benhiller)\ **Post date:** [May 18, 2007, 3:26pm UTC](https://www.ruby-forum.com/t/model-self-is-not-childs-parent/104436/4 "2007-05-18T15:26:24Z")
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 Y. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > > assert p.equal?(p.children[0].parent) > > following the relationships down and back up again will give you a – Amos K. Ramped Media USPS Programmer/Analyst St. Louis, MO Looking for something to do? Visit [ImThere.com](http://ImThere.com) ---
**Author:** ![benhiller](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/8e7dd6/32.png) [@benhiller](https://www.ruby-forum.com/u/benhiller)\ **Post date:** [May 18, 2007, 11:27pm UTC](https://www.ruby-forum.com/t/model-self-is-not-childs-parent/104436/5 "2007-05-18T23:27:27Z")
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 H. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) ---
**Author:** ![benhiller](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/8e7dd6/32.png) [@benhiller](https://www.ruby-forum.com/u/benhiller)\ **Post date:** [May 19, 2007, 12:07am UTC](https://www.ruby-forum.com/t/model-self-is-not-childs-parent/104436/6 "2007-05-19T00:07:32Z")
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 ---
**Author:** ![benhiller](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/8e7dd6/32.png) [@benhiller](https://www.ruby-forum.com/u/benhiller)\ **Post date:** [May 19, 2007, 1:13am UTC](https://www.ruby-forum.com/t/model-self-is-not-childs-parent/104436/7 "2007-05-19T01:13:16Z")
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 ---
**Author:** ![benhiller](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/8e7dd6/32.png) [@benhiller](https://www.ruby-forum.com/u/benhiller)\ **Post date:** [May 18, 2007, 11:39pm UTC](https://www.ruby-forum.com/t/model-self-is-not-childs-parent/104436/8 "2007-05-18T23:39:28Z")
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. ---
**Author:** ![benhiller](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/8e7dd6/32.png) [@benhiller](https://www.ruby-forum.com/u/benhiller)\ **Post date:** [May 22, 2007, 8:06pm UTC](https://www.ruby-forum.com/t/model-self-is-not-childs-parent/104436/9 "2007-05-22T20:06:13Z")
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, #\ is exactly equal to #\ 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 > **[Equality in Ruby - == eql? and equal?](http://www.wellho.net/mouth/985_Equality-in-Ruby-eql-and-equal-.html)** > > Equality in Ruby - == eql? and equal? On May 17, 3:13 pm, Ben H. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)