Rails - Jul 2010 - :dependent => :destroy with nonstandard :foreign_key?

Assume the following contrived (but plausible) geometric model in which
every Edge has two Vertices, every Vertex has many edges.
====  create_table "vertices", :force => true do |t|
    t.float    "x"
    t.float    "y"
    t.float    "z"
  end

  create_table "edges", :force => true do |t|
    t.integer  "vertex_a_id"
    t.integer  "vertex_b_id"
  end
====When we destroy a Vertex, we want all of its attached Edges to be
destroyed as well:
====  class Vertex << ActiveRecord::Base
    has_many :edges, :dependent => :destroy
  end

  class Edge << ActiveRecord::Base
    belongs_to :vertex_a, :class_name => ''Vertex'',
:foreign_key =>
''vertex_a_id''
    belongs_to :vertex_b, :class_name => ''Vertex'',
:foreign_key =>
''vertex_b_id''
  end
====Not surprisingly, the above associations won''t work when you try to
delete a vertex:
====  >> va = Vertex.create
  => #<Vertex id: 1...>
  >> vb = Vertex.create
  => #<Vertex id: 2...>
  >> e = Edge.create(:vertex_a => va, :vertex_b => vb)
  => #<Edge id: 1, vertex_a_id: 1, vertex_b_id: 2...>
  >> vb.destroy
  => ActiveRecord::StatementInvalid: Mysql::Error: Unknown column
''edges.vertex_id'' in ''where clause'': SELECT
* FROM `edges` WHERE
(`edges`.vertex_id = 2)
====... since Edge does not have a column named ''vertex_id''.

So: What''s the Rails-approved technique for destroying all Edges when
you destroy their Vertex, regardless of the Vertex''s role (vertex_a or
vertex_b)?

TIA.

- ff
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So I know it''s bad form to answer your own question, but I realized
this
must be a job for callbacks, so I RTFM''d.  My new definition for Vertex
is:

class Vertex < ActiveRecord::Base
  before_destroy do |record|
    Edge.destroy_all "vertex_a_id = #{record.id}"
    Edge.destroy_all "vertex_b_id = #{record.id}"
  end
end

It appears to do exactly what you''d expect: deleting a Vertex deletes 
all connected Edges.

Note that I also deleted the ":has_many edges" association for the
same
reason: Edge lacks the vertex_id field that AR depends upon to make the 
association, so you can''t ask for "va.edges()".  But
it''s simple enough
to mimic its functionality, as in:

class Vertex
  def edges
    Edge.find_all_by_vertex_a_id(self.id) + 
Edge.find_all_by_vertex_b_id(self.id)
  end
end

Thanks for listening...
- ff
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Fearless Fool wrote:
> So I know it''s bad form to answer your own question, but I
realized this
> must be a job for callbacks, so I RTFM''d.  My new definition for
Vertex
> is:
> 
> class Vertex < ActiveRecord::Base
>   before_destroy do |record|
>     Edge.destroy_all "vertex_a_id = #{record.id}"
>     Edge.destroy_all "vertex_b_id = #{record.id}"
>   end
> end
> 
> It appears to do exactly what you''d expect: deleting a Vertex
deletes
> all connected Edges.
Great!  You can probably unify those into one query and get better 
performance at the expense of a bit of Rails magic.
> 
> Note that I also deleted the ":has_many edges" association for
the same
> reason: Edge lacks the vertex_id field that AR depends upon to make the 
> association, so you can''t ask for "va.edges()".  But
it''s simple enough
> to mimic its functionality, as in:
> 
> class Vertex
>   def edges
>     Edge.find_all_by_vertex_a_id(self.id) + 
> Edge.find_all_by_vertex_b_id(self.id)
>   end
> end
...which you can now probably use a variant of in :finder_sql to restore 
your has_many :edges.  Again, you may be able to refactor it into one 
query.
> 
> Thanks for listening...
> - ff
Best,
--
Marnen Laibow-Koser
http://www.marnen.org
marnen-sbuyVjPbboAdnm+yROfE0A@public.gmane.org
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Resurrecting an old thread because it has a useful lesson:

Marnen Laibow-Koser wrote in post #927576:
> Fearless Fool wrote:
>> [stuff about a before_destroy method using two destroy_all calls]
>
> Great!  You can probably unify those into one query and get better
> performance at the expense of a bit of Rails magic.
>
>> [stuff about an edges method that concatenates two arrays]
>
> ...which you can now probably use a variant of in :finder_sql to restore
> your has_many :edges.  Again, you may be able to refactor it into one
> query.
So as usual, Marnen is right (if not ahead of his time).  Using the
scoping constructs in Rails 3.0, this all becomes elegant and simple:

  class Edge << ActiveRecord::Base
    belongs_to :vertex_a, :class_name => ''Vertex'',
:foreign_key =>
''vertex_a_id''
    belongs_to :vertex_b, :class_name => ''Vertex'',
:foreign_key =>
''vertex_b_id''
    scope :connected_to, lambda (vertex) {
      where("edges.vertex_a_id = ? OR edges.vertex_b_id = ?",
vertex.id,
vertex.id
    }
  end

  class Vertex < ActiveRecord::Base
    # caution: we are using delete_all for efficiency
    before_destroy { |vertex| Edge.connected_to(vertex).delete_all }
  end

This generates exactly the compact SQL that you''d expect, both for
finding:

  >> Edge.connected_to(v1)
  => SELECT "edges.*" FROM "edges" WHERE
(edges.vertex_a_id = 2 OR
edges.vertex_b_id = 2)

and for deleting:

  >> Edge.connected_to(v1).delete_all
  => DELETE FROM "edges" WHERE (edges.vertex_a_id = 2 OR
edges.vertex_b_id = 2)

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On 12 August 2011 18:35, Fearless Fool
<lists-fsXkhYbjdPsEEoCn2XhGlw@public.gmane.org>
wrote:
> Resurrecting an old thread because it has a useful lesson:
>
> Marnen Laibow-Koser wrote in post #927576:
>> [snip]
O Marnen, Marnen! wherefore art thou Marnen?

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On Aug 12, 2011, at 4:07 PM, Colin Law wrote:
> On 12 August 2011 18:35, Fearless Fool
<lists-fsXkhYbjdPsEEoCn2XhGlw@public.gmane.org> wrote:
>> Resurrecting an old thread because it has a useful lesson:
>>
>> Marnen Laibow-Koser wrote in post #927576:
>>> [snip]
>
> O Marnen, Marnen! wherefore art thou Marnen?
>
Fun fact: "wherefore" means "why". [1]

I wonder where Marnen went, but I never wondered why he was Marnen. I  
did appreciate his unique brand of scorched earth and incredibly  
precise answers.

Walter

1. In the famous quote, Juliet is wondering *why* Romeo is named the  
way he is; because of his name, she cannot be allowed to love him. Her  
logic, typical for Shakespeare women, is quite sound: "A rose by any  
other name would smell as sweet..."

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Walter Davis wrote in post #1016477:
> On Aug 12, 2011, at 4:07 PM, Colin Law wrote:
>
>> On 12 August 2011 18:35, Fearless Fool
<lists-fsXkhYbjdPsEEoCn2XhGlw@public.gmane.org> wrote:
>>> Resurrecting an old thread because it has a useful lesson:
>>>
>>> Marnen Laibow-Koser wrote in post #927576:
>>>> [snip]
>>
>> O Marnen, Marnen! wherefore art thou Marnen?
>>
>
> Fun fact: "wherefore" means "why". [1]
Indeed.
>
> I wonder where Marnen went, but I never wondered why he was Marnen. I
> did appreciate his unique brand of scorched earth and incredibly
> precise answers.
Um, thanks...?  I don''t know whether to feel flattered or put down. :D

Anyway, I''m still around.  I just haven''t been posting much. 
I went on
a self-imposed vacation from posting back in January (because I was 
getting too upset to respond as nicely as I wanted to!), and never got 
back into the habit.  I''ll probably be a more regular contributor again
in the near future.
>
> Walter
>
> 1. In the famous quote, Juliet is wondering *why* Romeo is named the
> way he is; because of his name, she cannot be allowed to love him. Her
> logic, typical for Shakespeare women, is quite sound: "A rose by any
> other name would smell as sweet..."
Yup.

Best,
--
Marnen Laibow-Koser
http://www.marnen.org
marnen-sbuyVjPbboAdnm+yROfE0A@public.gmane.org

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On Aug 28, 2011, at 6:23 PM, Marnen Laibow-Koser wrote:
> Um, thanks...?  I don''t know whether to feel flattered or put
down. :D
I hope you feel flattered, at least that was what I was aiming for. I  
did very much enjoy your posts, and learned quite a lot from them. I  
hope you can find a middle path between saint-like "suffer fools  
gladly" and and "getting too upset" to start answering again.
I''m sure
you have quite a lot more to teach me, and others here.

Walter

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