Rails - Oct 2008 - Iterating through all model records without pre-instantiating them

Hi,

I''m writing a migration script which has to, well, actually migrate
all data from table A to table B, but I need to access the application
logic in model A for each row migrated. I could do this:

MyModel.all.each do |instance|
  # do stuff...
end

but as far as I know that would pre-instantiate all records in memory
beforehand, which could be *really bad* with the amount of records I''m
dealing with, and I only really need to work with a record at a time.
An alternative I''m currently using is:

execute("SELECT * FROM table_a").each_hash do |row|
  instance = MyModel.new(row)
end

but I believe this is also sub-optimal (especially since some
attributes are attr_protected, therefore I''m having to take care of
those manually, and other reasons).

Is there any obvious/elegant way of doing this I''m missing?

Thanks,

-Pedro

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> I''m writing a migration script which has to, well, actually
migrate
> all data from table A to table B, but I need to access the application
> logic in model A for each row migrated. I could do this:
>
> MyModel.all.each do |instance|
>   # do stuff...
> end
>
> but as far as I know that would pre-instantiate all records in memory
> beforehand, which could be *really bad* with the amount of records
I''m
> dealing with, and I only really need to work with a record at a time.
Hi,

Have you thought of using limit and a offset with a regular find?
(below is the code but I didn''t check if it''s valid)

current_offset = 0
limit = 100

begin
  result = MyModel.find(:all, :limit=>limit, :offset=>current_offset)
  #some operations
  current_offset += limit
end while result.size == 0


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Nice one, thanks!

On Tue, Oct 28, 2008 at 7:22 PM, reHa
<preszke-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:
>
>> I''m writing a migration script which has to, well, actually
migrate
>> all data from table A to table B, but I need to access the application
>> logic in model A for each row migrated. I could do this:
>>
>> MyModel.all.each do |instance|
>>   # do stuff...
>> end
>>
>> but as far as I know that would pre-instantiate all records in memory
>> beforehand, which could be *really bad* with the amount of records
I''m
>> dealing with, and I only really need to work with a record at a time.
>
> Hi,
>
> Have you thought of using limit and a offset with a regular find?
> (below is the code but I didn''t check if it''s valid)
>
> current_offset = 0
> limit = 100
>
> begin
>  result = MyModel.find(:all, :limit=>limit, :offset=>current_offset)
>  #some operations
>  current_offset += limit
> end while result.size == 0
>
>
> >
>
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