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Peter Michaux

2007-Jun-30 18:26 UTC

transactions with two models

Hi,

In Agile Web Development with Rails section on Transactions (p 381)
there are two main examples.

1) making changes in two records in the same database table
2) making changes in two records each in a different *database*

I want to do what is in between: changes in two records each in a
different table of the same database. Suppose they are apples and
oranges tables.

How do I write this?

Do I need to be as elaborate as this and will it work?

apple = Apple.find(:first)
orange = Orange.find(:first)

Apple.transaction(apple) do
  Orange.transaction(orange) do
    apple.update_attributes(params[:apple])
    orange.update_attributes(params[:orange])
  end
end

Or can I just write something much simpler like this?

Apple.transaction(apple, orange) do
  apple.update_attributes(params[:apple])
  orange.update_attributes(params[:orange])
end

Will the above be a problem since orange is not an Apple?

In the rails docs there is something even simpler

transaction do
  apple.update_attributes(params[:apple])
  orange.update_attributes(params[:orange])
end

but I think this version won''t restore the apple and orange instances
back to their previous state if the transaction fails.


What I don''t understand is the class name that preceeds
".transaction". Does that class name make a transaction for just that
table or with the entire *database* associated with that table?

Any help greatly appreciated.

Thanks,
Peter

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Peter Michaux

2007-Jul-02 22:25 UTC

head link

transactions with two models

Hi,

In Agile Web Development with Rails section on Transactions (p 381)
there are two main examples.

1) making changes in two records in the same database table
2) making changes in two records each in a different *database*

I want to do what is in between: changes in two records each in a
different table of the same database. Suppose they are apples and
oranges tables.

How do I write this?

Do I need to be as elaborate as this and will it work?

apple = Apple.find(:first)
orange = Orange.find(:first)

Apple.transaction(apple) do
 Orange.transaction(orange) do
   apple.update_attributes(params[:apple])
   orange.update_attributes(params[:orange])
 end
end

Or can I just write something much simpler like this?

Apple.transaction(apple, orange) do
 apple.update_attributes(params[:apple])
 orange.update_attributes(params[:orange])
end

Will the above be a problem since orange is not an Apple?

In the rails docs there is something even simpler

transaction do
 apple.update_attributes(params[:apple])
 orange.update_attributes(params[:orange])
end

but I think this version won''t restore the apple and orange instances
back to their previous state if the transaction fails.


What I don''t understand is the class name that precedes
".transaction". Does that class name make a transaction for just that
table or with the entire *database* associated with that table?

Any help greatly appreciated.

Thanks,
Peter


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Michael Schuerig

2007-Jul-02 23:58 UTC

head link

Re: transactions with two models

On Tuesday 03 July 2007, Peter Michaux wrote:
> Do I need to be as elaborate as this and will it work?
>
> apple = Apple.find(:first)
> orange = Orange.find(:first)
>
> Apple.transaction(apple) do
>  Orange.transaction(orange) do
>    apple.update_attributes(params[:apple])
>    orange.update_attributes(params[:orange])
>  end
> end
>
> Or can I just write something much simpler like this?
>
> Apple.transaction(apple, orange) do
>  apple.update_attributes(params[:apple])
>  orange.update_attributes(params[:orange])
> end
Both versions are equivalent as ActiveRecord doesn''t do nested 
transactions. Transaction blocks only open a transaction if one is not 
already open.
> Will the above be a problem since orange is not an Apple?
>
> In the rails docs there is something even simpler
>
> transaction do
>  apple.update_attributes(params[:apple])
>  orange.update_attributes(params[:orange])
> end
>
> but I think this version won''t restore the apple and orange
instances
> back to their previous state if the transaction fails.
True, but object-level transactions are deprecated and will disappear 
some time in the future. From this, I take it, that they haven''t been 
widely used. See the API docs for more info.
> What I don''t understand is the class name that precedes
> ".transaction". Does that class name make a transaction for just
that
> table or with the entire *database* associated with that table?
The class has no more significance than that it is used to access the 
database connection. Commonly all classes share a connection to the 
same database, thus it doesn''t really matter which class you use here. 
You could even write ActiveRecord::Base.transaction { ... }.

In cases where your model classes *are* connected to different 
databases, you are out of luck regarding transactions, anyway, as 
ActiveRecord doesn''t ensure transactional consistency among multiple 
databases; there is no support for 2-Phase-Commit.

Michael

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

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Jacob Atzen

2007-Jul-17 07:29 UTC

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Re: transactions with two models

Peter Michaux wrote:> I want to do what is in between: changes in two records each in a
> different table of the same database. Suppose they are apples and
> oranges tables.
> 
[...]> Or can I just write something much simpler like this?
> 
> Apple.transaction(apple, orange) do
>   apple.update_attributes(params[:apple])
>   orange.update_attributes(params[:orange])
> end
This should work. Though the usage of instances as parameters to the 
transaction method is deprecated - check the docs.
> Will the above be a problem since orange is not an Apple?
Yes, the transaction affects the database connection not the individual 
tables.
> In the rails docs there is something even simpler
> 
> transaction do
>   apple.update_attributes(params[:apple])
>   orange.update_attributes(params[:orange])
> end
> 
> but I think this version won''t restore the apple and orange
instances
> back to their previous state if the transaction fails.
Because this behaviour is deprecated.

You can check your development log to see exactly what queries are 
executed, that should show you the transactions also.

-- 
Cheers,
- Jacob Atzen

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