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photo matt

2007-Feb-07 12:30 UTC

don''t delete if has_many > 0

My two models are ProductType and Product.  ProductType has_many
Products.

ProductType.destroy should fail and set an error if products.count > 0.

Before I re-invent the wheel, there must be some rails way of
approaching this problem.  Any thoughts?

Thanks!

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Steve Bartholomew

2007-Feb-07 13:01 UTC

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Re: don''t delete if has_many > 0

Try something like this in your model:

def before_destroy
  raise "Type as associated products" if products.count > 0
end

Steve


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Bill Walton

2007-Feb-07 14:11 UTC

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Re: don''t delete if has_many > 0

photo matt wrote:
> My two models are ProductType and Product.
> ProductType has_many Products.
>
> ProductType.destroy should fail and set an error
> if products.count > 0.
>
> Before I re-invent the wheel, there must be some
> rails way of approaching this problem.  Any thoughts?
If you''ve (1) included belongs_to ProductType in your Products model,
and
(2) included ProductType_id as a foreign key in your Products table, then 
what you''re looking for is the default behavior.

hth,
Bill 



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photo matt

2007-Feb-08 01:10 UTC

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Re: don''t delete if has_many > 0

Bill and Steve,

Thank you for your responses.

Steve--your suggestion simply raises an exception, which is not the most
elegant solution.  I''d rather use an error that can actually be
returned.

Bill -- you may be right, but my application is not working like that.
First, maybe I need a little lesson on naming things....

My ProductType model pulls from a database table called product_types.
My Product model pulls from a table named products, and has the
following association:    belongs_to :product_type
The products table also has a column named product_type_id.

Is my problem that I''ve used underscores in my table names?  I thought
that was the proper table naming convention for names that have two
words.

Thanks,

Matt




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Bill Walton

2007-Feb-08 03:53 UTC

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Re: don''t delete if has_many > 0

Hi Matt,

photo matt wrote:

First, thank you for signing your post.  Maybe I''m just getting old but
I
like to know with whom I''m conversing.
> my application is not working like that.
Then it''s broke.  But then you already knew that ;-)

I''ll apologize up front for the shortness of my response.  I
don''t have much
time at the moment and hope you won''t be offended.

Here''s what I''d do if I had 20 minutes and your code. 
It''s what I _always_
do when I''m going to try something new or having a problem.

Do NOT use any of the Rails magic you don''t absolutely have to except
for
scaffolding.  The scaffold code works.  Always.  Unless you''ve screwed
up
something it depends on.  And that''s what you need to find out.  If
you''re
using migrations, don''t for this.  Because that might be where
you''re
problem is.  Use a script.  Same principle applies across the board.  Focus 
on what you''re trying to discover and don''t include ANYTHING
that might
obscure that.

1) Set up a database named ''sandbox'' (if you don''t
already have one)
2) Create the tables you need to investigate the thing you''re
interested in.
In your case you need two tables.  Use the same convention you''re
already
using.  Other than pluralization, Rails doesn''t give a damn whether you
use
underscores / CamelCase or not.  Use the scaffolding to learn.  The 
product_types table should only have two fields: id and a text field named 
whatever you''ve got it named in the app you''re having a
problem with.  The
products table should only have three fields: id, product_type_id, and a 
text field.
3) Scaffold your sandbox app on both the product and product_type models.
4) Add the relations to your models.
5) Add a product type
6) Add a product
7) Try to delete the product type.  You should get an error.  If you
don''t,
the problem''s in your setup / config.  If you do, the
problem''s in your
(other) code.

I''m sorry I can''t offer more right now.  I''ll be back
in 8-10 hours.  Let us
know how it goes.

Best regards,
Bill 



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photo matt

2007-Feb-09 00:28 UTC

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Re: don''t delete if has_many > 0

Bill,

Thank you for your suggestions.  I managed to reproduce the same 
behavior in a sandbox.  I''ve also searched the documentation and I
can''t
seem to find where it says that this is the default behavior.

This definitely can be enforced at the database level, and maybe it 
should be done there?

Thanks,

Matt


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Bill Walton

2007-Feb-09 02:45 UTC

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Re: don''t delete if has_many > 0

Hi Matt,

photo matt wrote:
> I managed to reproduce the same behavior in
> a sandbox.
Excellent!  Please post your sandbox code.  Including your SQL script.
> I''ve also searched the documentation and I can''t
> seem to find where it says that this is the default
> behavior.
Not surprising.  It''s an implied understanding about the way databases
work
You may well have discovered and reported a bug!

Good on ya, bud!  And even if it''s not a bug, we''ll all learn
something.

Best regards,
Bill 



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photo matt

2007-Feb-09 03:28 UTC

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Re: don''t delete if has_many > 0

Well, after a bit more research, I''m pretty convinced that this just 
isn''t the way rails works.

According to Agile Web Development with Rails, "Notice that this table 
has two foreign keys. Each row in the line_items table is associated 
both with an order and with a product. Unfortunately, Rails migrations 
dont provide a database-independent way to specify these foreign key
constraints, so we had to resort to executing native DDL statements (in 
this
case, those of MySQL)."

Furthermore, the has_many relationship has a :dependent attribute that 
can be set to :destroy_all, :delete_all, or :nullify.  It seems to me 
that it should have a :delete_restrict option, but it doesn''t.

If you are using InnoDB tables on MySQL, you can use database level 
foreign key constraints that will protect the dependent rows from being 
orphaned.  I''ll do that, and I am reasonably confident it will work. 
But it seems to me that Rails should have some method of preventing rows 
from being orphaned.

http://dev.mysql.com/doc/refman/5.0/en/innodb-foreign-key-constraints.html

Thanks,

Matt

Bill Walton wrote:> Hi Matt,
> 
> photo matt wrote:
> 
>> I managed to reproduce the same behavior in
>> a sandbox.
> 
> Excellent!  Please post your sandbox code.  Including your SQL script
> 
>> I''ve also searched the documentation and I can''t
>> seem to find where it says that this is the default
>> behavior.
> 
> Not surprising.  It''s an implied understanding about the way
databases
> work
> You may well have discovered and reported a bug!
> 
> Good on ya, bud!  And even if it''s not a bug, we''ll all
learn something.
> 
> Best regards,
> Bill

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Bill Walton

2007-Feb-09 03:59 UTC

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Re: don''t delete if has_many > 0

photo matt wrote:
> Unfortunately, Rails migrations don''t provide a
> database-independent way to specify these foreign key
> constraints, so we had to resort to executing native
> DDL statements (in this case, those of MySQL)."
Which is precisely why I said, in my earlier response:> If you''re using migrations, don''t for this.  Because that
> might be where you''re problem is.  Use a script.
" Do I contradict myself? Very well, then I contradict myself, I am large,
I
contain multitudes. "
Walt Whitman

Congrats on your learning, though.  Good post.  You''ve undoubtedly
helped
someone who will follow your footsteps.  I hope you see that as a Good 
Thing.

Best regards,
Bill 



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Bill Walton

2007-Feb-09 04:09 UTC

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Re: don''t delete if has_many > 0

> " Do I contradict myself? Very well, then I contradict myself, I am
large,
> I
> contain multitudes. "
> Walt Whitman
The allusion was to Rails. 



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photo matt

2007-Feb-10 13:59 UTC

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Re: don''t delete if has_many > 0

Here is my ruby solution to this problem.

In the product type model, implement the before_destroy method.  This 
gives us a chance to cancel the destroy by returning false.
def before_destroy
  if products.count() > 0
    return false
  end
end

In the product types controller, check the return value of destroy, and 
if it is false, output an error message.

def destroy
  if !ProductType.find(params[:id]).destroy
    flash[:notice] = "Cannot delete this product type until there are no 
products with this product type."
  end
  redirect_to :action => ''list''
end

It seems to me that at least the part in the model should be an option 
on the :has_many declaration.  The part I''m not so sold on, is
shouldn''t
that error message that I''m setting in the controller be set somewhere 
in the model?  But then, how would the View ever get that error message?

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Rails - Feb 2007 - don't delete if has_many > 0

don''t delete if has_many > 0

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