From: Robert Klemme <shortcutter@...>
Date: 2012-02-08T07:04:01+09:00
Subject: Re: Appending to an Array

On Tue, Feb 7, 2012 at 10:27 PM, Anthony Simonelli
<asimonelli01@gmail.com> wrote:
> I am querying a database using DBI and ODBC.  After I run the execute, I
> want to grab each row as an array, and add it to another array, essentially
> creating an Array of Arrays.  I've tried this two ways with the same result:
>
> sth = dbh.execute("SELECT * FROM SOMETABLE")
> rows = sth.fetch_all
>
> and
>
> rows = sth.fetch do |row|
>    rows << row
> end
>
> The result for both is that the last row is repeated in the array however
> many times there are rows.  Why isn't it appending correctly?

Probably because the DB driver optimizes object allocation and reuses
the same instance.  Then this is what happens:

irb(main):006:0> rows = []
=> []
irb(main):007:0> a = %w{a b c d}
=> ["a", "b", "c", "d"]
irb(main):008:0> rows << a
=> [["a", "b", "c", "d"]]
irb(main):009:0> a[2] = 99
=> 99
irb(main):010:0> rows << a
=> [["a", "b", 99, "d"], ["a", "b", 99, "d"]]

The phenomenon is called "aliasing".

You can solve it by copying row Arrays.

rows = []

sth.fetch do |row|
  rows << row.to_a # if rows is not an Array
  rows << row.dup # if rows is an Array
end

Kind regards

robert


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