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Michael Schuerig

2008-May-20 16:21 UTC

Finding parents with red-haired children?

Here''s the basic situation

  class Person < ActiveRecord::Base
    acts_as_tree
    belongs_to :hair_color
    validates_presence_of :name
  end

  class HairColor < ActiveRecord::Base
    validates_presence_of :name
  end

What I want to achieve:

I need another association on Person that returns only the red-haired 
children. In my special case[*] it has to be an association, an 
association extension, named scope or similar won''t do. In particular, 
this association has to work when listing all persons and their 
red-headed children and possibly with a filter condition.

A first stab looks like this

  class Person < ActiveRecord::Base
    ...
    has_many :redheads, :class_name => ''Person'',
      :foreign_key => :parent_id,
      :include => :hair_color, 
      :conditions => "hair_colors.name = ''red''"
  end

This works indeed for a case like this

  p = Person.find(:first)
  p.redheads

however, it doesn''t work for

  parents = Person.find(:all, :include => :redheads)

In that case, the secondary assocation (redheads.hair_color) doesn''t 
even seem to be included. Adding it manually like this

  parents = Person.find(:all, :include => [:redheads, :hair_color])

doesn''t help because the "scoping" is wrong: hair_color is
joined on the
parents, not the children.

So, is there a way to make ActiveRecord do what I need without resorting 
to hand-written SQL?

Michael


[*] I already have the infrastructure to display and filter lists of 
objects with associated objects *one-level* deep. E.g., for a list of 
persons I could define a column { :column =>
''hair_color.name'', :title
=> ''Hair color'' }. I''d like to specify a column
such as { :column
=> ''redheads.name'', :title =>
''Redheads'' }.

-- 
Michael Schuerig
mailto:michael-q5aiKMLteq4b1SvskN2V4Q@public.gmane.org
http://www.schuerig.de/michael/

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Ruby Freak

2008-May-20 16:29 UTC

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Re: Finding parents with red-haired children?

What about Red-Headed step children?

sorry, couldn''t help it

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Frederick Cheung

2008-May-20 16:52 UTC

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Re: Finding parents with red-haired children?

On 20 May 2008, at 17:21, Michael Schuerig wrote:>
> however, it doesn''t work for
>
>  parents = Person.find(:all, :include => :redheads)
>
> In that case, the secondary assocation (redheads.hair_color)
doesn''t
> even seem to be included. Adding it manually like this
>
>  parents = Person.find(:all, :include => [:redheads, :hair_color])
  parents = Person.find(:all, :include => {:redheads => :hair_color})
?
Fred
>
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Ruby Freak

2008-May-20 17:30 UTC

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Re: Finding parents with red-haired children?

What about 2 steps

@parent_ids = find(:all, :conditions => "hair_colors.name =
''red''"}
@red_head_parents = find(:all, :conditions => ":id IN
@parent_ids.uniq''"}

I prolly mucked the syntax, but ...

On May 20, 9:52 am, Frederick Cheung
<frederick.che...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:> On 20 May 2008, at 17:21, Michael Schuerig wrote:
>
>
>
> > however, it doesn''t work for
>
> >  parents = Person.find(:all, :include => :redheads)
>
> > In that case, the secondary assocation (redheads.hair_color)
doesn''t
> > even seem to be included. Adding it manually like this
>
> >  parents = Person.find(:all, :include => [:redheads, :hair_color])
>
>   parents = Person.find(:all, :include => {:redheads => :hair_color})
> ?
> Fred
>
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Ruby Freak

2008-May-20 17:48 UTC

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Re: Finding parents with red-haired children?

lets try that again

redheads = Person.find(:all, :conditions => "hair_colors.name
''red''"}
parent_ids = redheads .inject([]) do {|parent_list,
redheads.parent_id| parent_list<<redheads.parent_id}
red_head_parents = Person.find(:all, :conditions => ":id IN
parent_ids''"}

And I still prolly mucked it up
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Michael Schuerig

2008-May-20 18:05 UTC

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Re: Finding parents with red-haired children?

On Tuesday 20 May 2008, Frederick Cheung wrote:> On 20 May 2008, at 17:21, Michael Schuerig wrote:
> > however, it doesn''t work for
> >
> >  parents = Person.find(:all, :include => :redheads)
> >
> > In that case, the secondary assocation (redheads.hair_color)
> > doesn''t even seem to be included. Adding it manually like
this
> >
> >  parents = Person.find(:all, :include => [:redheads, :hair_color])
>
>   parents = Person.find(:all, :include => {:redheads => :hair_color})
> ?
No, unfortunately not. I have a test dataset with 20 persons -- 4 
parents + 16 children -- and a "random" assignment of hair colors.
>> ps = Person.find(:all, :include => { :redheads => :hair_color })
>> ps.map { |p| p.redheads.map(&:hair_color).map(&:name) }=> [["black", "black", "blonde",
"brown"],
["brown", "black", "red", "blonde"], 
["blonde", "red", "brown", "brown"], 
["brown", "black", "brown", "blonde"],
[], [], [], [], [], [], [], [],
[], [], [], [], [], [], [], []]

When I include the parent''s hair color, too, the result deteriorates
>> ps = Person.find(:all, :include => [:hair_color, { :redheads 
=> :hair_color }])>> ps.map { |p| p.redheads.map(&:hair_color).map(&:name) }=> [[], [], [], ["brown", "black", "brown",
"blonde"], [], [], [], [],
[], [], [], [], [], [], [], [], [], [], [], []]

I''m not too fond of dissecting the generated SQL and especially why it 
was generated like that. From a somewhat superficial vantage point I''d 
say that the cause of the trouble lies with joining at the correct 
level, that is children, not parents. IIRC, the association code was 
slated for a thorough revision for Rails 3.0, I may just wait for it.

Michael

-- 
Michael Schuerig
mailto:michael-q5aiKMLteq4b1SvskN2V4Q@public.gmane.org
http://www.schuerig.de/michael/

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Michael Schuerig

2008-May-20 18:31 UTC

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Re: Finding parents with red-haired children?

On Tuesday 20 May 2008, Ruby Freak wrote:> lets try that again
>
> redheads = Person.find(:all, :conditions => "hair_colors.name >
''red''"}
> parent_ids = redheads .inject([]) do {|parent_list,
> redheads.parent_id| parent_list<<redheads.parent_id}
> red_head_parents = Person.find(:all, :conditions => ":id IN
> parent_ids''"}
>
> And I still prolly mucked it up
Yes, on two counts. First, I''m not concerned with red-haired parents, 
but with their children. Second, I want to retrieve *all* parents with 
their red-haired children (if any) included.

As all this is going to be displayed in a list with optional filter 
conditions, I may need to filter by the parent''s name or hair color as 
well as by a child''s name.

Just consider the case where you have a paginated list of all people in 
your db and you want to filter it down to only the red-haired people 
who have a red-haired child named John.

Michael

-- 
Michael Schuerig
mailto:michael-q5aiKMLteq4b1SvskN2V4Q@public.gmane.org
http://www.schuerig.de/michael/

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Frederick Cheung

2008-May-20 19:48 UTC

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Re: Finding parents with red-haired children?

On 20 May 2008, at 19:05, Michael Schuerig wrote:
>
> On Tuesday 20 May 2008, Frederick Cheung wrote:
>> On 20 May 2008, at 17:21, Michael Schuerig wrote:
>>> however, it doesn''t work for
>>>
>>> parents = Person.find(:all, :include => :redheads)
>>>
>>> In that case, the secondary assocation (redheads.hair_color)
>>> doesn''t even seem to be included. Adding it manually like
this
>>>
>>> parents = Person.find(:all, :include => [:redheads,
:hair_color])
>>
>>  parents = Person.find(:all, :include => {:redheads =>
:hair_color})
>> ?
>
> No, unfortunately not. I have a test dataset with 20 persons -- 4
> parents + 16 children -- and a "random" assignment of hair
colors.
>
>>> ps = Person.find(:all, :include => { :redheads => :hair_color
})
>>> ps.map { |p| p.redheads.map(&:hair_color).map(&:name) }
> => [["black", "black", "blonde",
"brown"],
> ["brown", "black", "red",
"blonde"],
> ["blonde", "red", "brown",
"brown"],
> ["brown", "black", "brown",
"blonde"], [], [], [], [], [], [], [], [],
> [], [], [], [], [], [], [], []]
>
> When I include the parent''s hair color, too, the result
deteriorates
>
Ah yes, conditions on :include are a bit iffy and often give  
unexpected consequences. Rails 2.1 has a rewrite of eager loading but  
it may not help you

Could you do it the otherway round i.e. something like  
red_color.find :people, :include => :parent
That gives you all redheads with their parents. Then (can''t think of a
better verb) transpose/invert this to have it by parent.

This may be one of those times when writing some grungy sql is the way  
forward.

Fred
>>> ps = Person.find(:all, :include => [:hair_color, { :redheads
> => :hair_color }])
>>> ps.map { |p| p.redheads.map(&:hair_color).map(&:name) }
> => [[], [], [], ["brown", "black",
"brown", "blonde"], [], [], [], [],
> [], [], [], [], [], [], [], [], [], [], [], []]
>
> I''m not too fond of dissecting the generated SQL and especially
why it
> was generated like that. From a somewhat superficial vantage point
I''d
> say that the cause of the trouble lies with joining at the correct
> level, that is children, not parents. IIRC, the association code was
> slated for a thorough revision for Rails 3.0, I may just wait for it.
>
> Michael
>
> -- 
> Michael Schuerig
> mailto:michael-q5aiKMLteq4b1SvskN2V4Q@public.gmane.org
> http://www.schuerig.de/michael/
>
> >

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Michael Schuerig

2008-May-20 20:42 UTC

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Re: Finding parents with red-haired children?

On Tuesday 20 May 2008, Frederick Cheung wrote:> Could you do it the otherway round i.e. something like  
> red_color.find :people, :include => :parent
> That gives you all redheads with their parents. Then (can''t think
of
> a   better verb) transpose/invert this to have it by parent.
No, that doesn''t work because I want to list *all* persons but 
specifically include the red-haired children in the list. A more 
practical reason is that it does not fit with my current infrastructure 
that nicely handles all the easier cases.

Incidentally, I''m not preoccupied with redheads, apart from myself 
having a hair color in that general direction. It''s just that this 
somewhat contorted example perfectly fits my real problem.

Michael

-- 
Michael Schuerig
mailto:michael-q5aiKMLteq4b1SvskN2V4Q@public.gmane.org
http://www.schuerig.de/michael/

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frioux-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org

2008-May-21 05:37 UTC

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Re: Finding parents with red-haired children?

> No, that doesn''t work because I want to list *all* persons but
> specifically include the red-haired children in the list. A more
> practical reason is that it does not fit with my current infrastructure
> that nicely handles all the easier cases.
>
> Incidentally, I''m not preoccupied with redheads, apart from myself
> having a hair color in that general direction. It''s just that this
> somewhat contorted example perfectly fits my real problem.
Did you ever figure this one out?  I in a similar situation.
Basically I am trying to  fake inheritance with a belongs_to
relationship, which means that if I want to search by any of the
columns that come from the "parent" object I have to do it outside of
the database.

Anyway, if I figure anything out I''ll note it here.

-fREW
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Rails - May 2008 - Finding parents with red-haired children?

Finding parents with red-haired children?

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