Read my latest article: 8 things I look for in a Ruby on Rails app (posted Thu, 06 Jul 2017 17:59:00 GMT)

Active Record, I <3 U but I still trust my database server (a tiny bit more)

Posted by Fri, 19 Aug 2005 02:20:00 GMT

While working on a portion of my book, I found myself in ./script/console and was seeing some weird issues when I would use has_many and belongs_to.

Let’s take two simple models.

<pre>
class Order < ActiveRecord::Base
  belongs_to :customer
end

class Customer < ActiveRecord::Base
  has_many :orders
end
</pre>
</code>

After a few test records...

<code>
<pre>
test_dev=# SELECT * FROM customers;  
 id |      name      
----+----------------
  1 | Robby
  2 | Nigel
  3 | Linus
(3 rows)

test_dev=# SELECT * FROM orders;
 id | customer_id | amount 
----+-------------+--------
  1 |           1 |  12.00
  2 |           3 |  12.00
(2 rows)
</pre>
</code>

Nothing completely crazy going on, right?

<typo:code lang="ruby">
Loading development environment.
>> Customer.destroy(3)
=> {"name"=>"Linus", "id"=>"3"}
>>     

=# SELECT * FROM orders;
 id | customer_id | amount 
----+-------------+--------
  1 |           1 |  12.00
  3 |           3 |  12.00
(2 rows)

Wait a minute! I just deleted a customer with an id of 3!

So, what is wrong with this scenario? Can you think of any potential problems that could occur from data like this? The record has a customer_id for a customer that does not exist. This is why we have relational databases in the first place, right? :-)

Here is something that I learned today that I was unaware of. Active Record allows you to pass the has_method declaration the option :dependent.

class Customer < ActiveRecord::Base
  has_many :orders, :dependent => true
end

What is this option? Well, according to the AR documentation, “:dependent – if set to true all the associated object are destroyed alongside this object. May not be set if :exclusively_dependent is also set.”

In a nutshell, this works like ON DELETE CASCADE does in PostgreSQL. So, it will through and delete the orders associated with the customer that I was attempting to destroy.

Up until today, I hadn’t broken myself out of the habit of using the built-in constraints/triggers of PostgreSQL. So, as soon as I did, this issue came up and I learned about :dependent.

test_dev# \d orders
                                Table "public.orders" 
   Column    |     Type      |                       Modifiers                        
-------------+---------------+--------------------------------------------------------
 id          | integer       | not null default nextval('public.orders_id_seq'::text)
 customer_id | integer       | 
 amount      | numeric(10,2) | 
Indexes:
    "orders_pkey" PRIMARY KEY, btree (id)
Foreign-key constraints:
    "orders_customer_id_fkey" FOREIGN KEY (customer_id) REFERENCES customers(id)

test_dev=# ALTER TABLE orders DROP CONSTRAINT orders_customer_id_fkey;
ALTER TABLE 

RobbyOnRails:~/Programming/footest robbyrussell$ ./script/console 
Loading development environment.
>> cust = Customer.create(:name => 'Jim')
=> #<Customer:0x275373c @new_record_before_save=true, @new_record=false, @attributes={"name"=>"Jim", "id"=>5}, @errors=#<ActiveRecord::Errors:0x274fa88 @base=#<Customer:0x275373c ...>, @errors={}>>
>> cust.orders.create(:amount => '25.00')
=> #<Order:0x274991c @new_record=false, @attributes={"id"=>4, "amount"=>"25.00", "customer_id"=>5}, @errors=#<ActiveRecord::Errors:0x2746dfc @base=#<Order:0x274991c ...>, @errors={}>>
>>                

test_dev=# SELECT * FROM orders;
 id | customer_id | amount 
----+-------------+--------
  1 |           1 |  12.00
  3 |           4 |  29.00
  4 |           5 |  25.00
(3 rows)

As you can see, I put myself into the hands of Active Record when I ran the DROP CONSTRAINT. Then I tried running the code at the top of the post… and it didn’t work.

According to the docs, if you use :dependent => true, it should delete the foreign table records. If not, it should set the value of the foriegn key field to NULL in the foreign table.

Basically, perform these two SQL queries:
UPDATE orders SET customer_id = NULL WHERE customer_id = 17; 

DELETE FROM customers  WHERE id = 17;

Then, the records are still in the database for those orders, but the customer is deleted. There are arguments for and against doing this sort of thing… but the ability to have the option is always nice. In any event, Active Record would not run the first query,it was just deleting from the customers table. Without my constraint, no error would be returned from PostgreSQL and I started to get some bad data.

Imagine showing a list of orders and trying to display the customer name associated with an order that has no linking customer. Doh! If Active Record sets the customer_id to NULL we can at least have some logic to work with this without having to run some fun SQL queries to figure out which orders do and dont have customers. (we want our applications to have clean data!)

Anyhow… Was this a bug? Should Active Record know to update the records to NULL in this case? I figured that is should be handling this task, especially since it was handling cascading deletes when you passed :dependent => true.

However, I didn’t want to prematurely post a bug report, so I began asking around on #rubyonrails (irc.freenode.net). People made a bunch of suggestions as to how to work around it. I could add a before_destroy method in my model, track the bug down, (re)add an ON DELETE trigger to my table (hah), etc. So, I decided that I would see if I could track down what happens when has_many is used for a model upon #destroy.

After a while of digging and making some tests, I posted a patch and a bug report. (please disregard my first patch… it did not work! heh)

Now that I figured this out, I am going to happy add my constraints back to my tables and go back to playing around. This reminded me of a post I had a few months ago when I mentioned that I thought it was best to put some constraints and logic in the database. I also agree that constraints should be put in the abstraction layer, but we cannot always put all faith in our code either. A few levels of checks doesn’t hurt. :-)

This was a fun little riddle that I took on today. The moral of the story? If you have the ability to use the builtin referential integrity features of PostgreSQL and those other databases, it might be a good idea to do so. Things get overlooked, people login to the database in many ways, and from different programs.

UPDATE: DHH responded to this post and provided a link which discusses Application Database versus Integrated Database

It should be noted that there is an important distinction between the two methods. When I said, “Things get overlooked, people login to the database in many ways, and from different programs” I was basically describing Integration Database. However, I was also thinking of the possibility of someone opening up their MySQL or PostgreSQL GUI and manually removing a record in plain SQL. According to Application Database, the moment that you do that, you basically break this model and cannot expect your application to be fully responsible for the problems that may or may not occur. At this point, you would need to look at your application in terms of Integration Database. Please do correct me if I am wrong on this. :-)

However, with this scenario, my first attempt to move to relying on AR had a minor hiccup, but it was an easy enough fix.

By performing the following command, I am moving towards an Integration Database pattern and that should be recognized when taking this into consideration.

ALTER TABLE orders ADD CONSTRAINT orders_customers_id_fkey 
    FOREIGN KEY (customer_id) REFERENCES customers (id) MATCH FULL; 

Okay, back to work!

Once again. Use constraints! (if you can)

... and thanks to DHH for providing the link and motivating me to make a note of this in my entry.

Extending ActionController

Posted by Sun, 05 Jun 2005 18:33:00 GMT

In my pursuit yesterday to make add the desired functionality to ActionController (render string to string), I talked to bitsweat about this and he suggested that I just extend the class in my rails app.

So, I created a file in lib, called action_controller_ext.rb (a shim as he referred to it) and put this in the file:

require 'action_controller'

class ActionController::Base

  def render_string_to_string(text = nil, type = "rhtml") #:doc:
    add_variables_to_assigns
    @template.render_template(type, text) 
  end

end
Then in environment.rb, I changed:
require 'action_controller'
to
require 'action_controller_ext'

Now, I can move my clients app around dev/test/live servers and not need to modify the actual Rails source code. (still would be nice if it did that though)

Ruby FPDF on Ruby on Rails

Posted by Sat, 04 Jun 2005 14:24:00 GMT

3 comments Latest by sg Mon, 21 Aug 2006 09:57:06 GMT

I have been tinkering with Ruby FPDF for a client all night. I found the examples for it to lack in some real-world examples, so I have taken the example from the RubyOnRails wiki and added a bit more to it. I have added things like an image, links and made a generic letter template. (just an example). I didn’t get into the header/footer functions yet, may do that later.

If you are not sure how to use Ruby FPDF, check out the FPDF documentation.

Just download Ruby FPDF and unpack the archive in your libs directory.

To get Railst to load fpdf, I added this to environment.rb:

ADDITIONAL_LOAD_PATHS.concat %w(
  app
  app/models
[..snip...]
  lib/fpdf
).map { |dir| "#{RAILS_ROOT}/#{dir}" }.select { |dir| File.directory?(dir) }
At the top of my controller that uses this lib, I added:
require 'fpdf'

When I browse to this controller/method, I got this PDF to generate with this code.

  def pdf
    send_data gen_pdf, :filename => "robbyonrails-fpdf-test.pdf", :type => "application/pdf"
  end

 private
  def gen_pdf

    d = Date.today

    pdf=FPDF.new
    pdf.AddPage
    pdf.SetFont('Arial')
    pdf.SetFontSize(10)

    pdf.Image('/home/matchboy/logo2.jpg',10,8,86,0,'JPG')

    pdf.Cell(0,6, "PLANET ARGON", 0,1,'R')
    pdf.Cell(0,6, "2802 NE 57th Ave",0,1,'R')
    pdf.Cell(0,6, "Portland, OR 97213",0,0,'R')

    pdf.Ln
    pdf.Ln
    pdf.Write(5, "Jane Doe
123 ABC Street
Gilroy, CA 95020

#{d.month}/#{d.mday}/#{d.year}

Dear Jane Doe,

I just wanted to say...

Epsum factorial non deposit quid pro quo hic escorol. Olypian quarrels et gorilla congolium sic ad nauseum. Souvlaki ignitus carborundum e pluribus unum. Defacto lingo est igpay atinlay. Marquee selectus non provisio incongruous feline nolo contendre. Gratuitous octopus niacin, sodium glutimate. Quote meon an estimate et non interruptus stadium. Sic tempus fugit esperanto hiccup estrogen. Glorious

Cheers,

Robby Russell
          ")
    pdf.Ln
    pdf.Cell(0,6, "PLANET ARGON", 0,1,'L',0,'http://www.planetargon.com/')
    pdf.Output
  end

Once again, the output. (click to view PDF)

It’s nothing special, but it’s just an example of how you can add a bit more than ‘Hello World’ to the top of a PDF. I’m still working on figuring out all the x/y stuff. Maybe I will post a better tutorial in the future with headers and footers.

Until then… have fun!

Parsing a RSS Feed

Posted by Wed, 11 May 2005 21:44:00 GMT

2 comments Latest by Neil Chandler Thu, 04 Feb 2010 10:43:09 GMT

A friend was asking me how they could easily read a RSS feed and display the last x items in their rails project. This was my quick and dirty response.


require 'rss/2.0'
require 'open-uri'

class RssfeedController &lt; ApplicationController

  def index
    feed_url = 'http://www.planetrubyonrails.org/xml/rss'
    output = "<h1>My RSS Reader</h1>" 
    open(feed_url) do |http|
      response = http.read
      result = RSS::Parser.parse(response, false)
      output += "Feed Title: #{result.channel.title}<br />" 
      result.items.each_with_index do |item, i|
        output += "#{i+1}. #{item.title}<br />" if i &lt; 10  
      end  
    end
    render_text output
  end

end

Is there an easier way to do this with another RSS library? I figured that the simplest method would be to just use the standard library that comes with Ruby.

The Zen of Ruby on Rails

Posted by Mon, 11 Apr 2005 01:46:00 GMT

It’s 1:30PM PST, and I am about to devote the next several hours to programming with Ruby on Rails. I haven’t been able to spend much time the last several days as I have been wrapping up a project that was started in PHP. I ended up spending about 7 hours the other day Refactoring a good chunk of the code base, using many of the examples outlined in Fowler’s book (catalog). Derek Sivers suggested this book last winter when I had the privilege of meeting him and discussing programming a bit. The book has been on my desk more than any other book since I purchased it at Powells in December.

Part of me wished that there was a good implementation of Active Record in PHP, so I debated writing it, but came across this from January. The author seems to use Ruby on Rails. I might get around to playing with it soon and see if it’ll make my PHP work a little less gruesome. :-)

In other news, my clients Rails-based application is coming along nicely. grins more

I am probably going to order a few books this week to promote some more best practice approaches to OO. If anyone has some good suggestions, feel free to comment and share the titles.

Cheers

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