Rails action caching with querystring parameters

We can cache a particular action with its query string parameters in rails as follows :

caches_action :action_name, :cache_path => Proc.new { |c| c.params }

Use of "!~" operator in Ruby

"!~" can be used to have conditional statements to check if string doesn't match particular regular expression. So that whenever we want to check if particular string is not matching a regular expression we can avoid having negative conditional statements using "unless" like

unless "str" =~ /reg_exp/
 # do something
end

We can write this conditional statement as assertive condition as follows:

if "str" !~ /reg_exp/
 # do something
end

Interpolating Instance, Class level or Global variable in Ruby

When the expression to be interpolated into the string literal is simply a reference to a global, instance or class variable, then the curly braces may be omitted.

So the following should all work: 

@first_name = "Anil"
puts "#@first_name Galve"  #=> "Anil Galve"

@@first_name = "Anil"
puts "#@@first_name Galve" #=> "Anil Galve"

$first_name = "Anil"
puts "#$first_name Galve"  #=> "Anil Galve"

before_save vs before_validate in Ruby On Rails

If we want to manipulate data before saving into database we tend to use before_save for example if we want to trim users first name before saving to database we would normally use before_save callback and will do something like below:

before_save :trim_first_name

def trim_first_name
  first_name = first_name.strip
end

But there is gotcha into it, it will skip validation for updated first_name because before_save callback gets executed after validations. So lets say, if there uniqueness validation for first_name and if there is already user with first_name "anil" present in database and if user specify "anil " then that user will be inserted since later one has extra space and it skips that validation and before saving into database it strips the first_name and defeats the purpose of our validation.

Hence, whenever we want to manipulate data before saving into database its good idea to have before_validation callback instead of before_save callback.

Same can be achieved like below:

before_validation :trim_first_name

def trim_first_name
  first_name = first_name.strip
end

Tracking ajax requests in google analytics

Recently I had one requirement from client to capture even ajax requests in application for google analytics reports.

I changed google analytics account for capturing dynamic content and added following javascript code in application.js file and it worked.

$(function() {
// Log all jQuery AJAX requests to Google Analytics
$(document).ajaxSend(function(event, xhr, settings){
if (typeof _gaq !== "undefined" && _gaq !== null) {
_gaq.push(['_trackPageview', settings.url]);
}
});
});

Delete Vs. Destroy In Rails

I’ve been writing this web app that has the following models: Order, Recipient, Message. An order has many recipients and a recipient has many messages. The recipients are also dependent upon the order and the messages are dependent upon the recipient.

In Ruby code this looks like:
Order.rb

class Order < ActiveRecord::Base
has_many :recipients, :dependent => :destroy
has_many :messages, :through => :recipients
end

Recipient.rb

class Recipient < ActiveRecord::Base
belongs_to :order
has_many :messages, :dependent => :destroy
end

The idea here is that when I delete an order, I also delete any associated recipients and any associated messages. My controller looked like this:

def delete
Order.delete(params[:id])
end

There are a couple things wrong with this. The first and most important thing is that when I delete a row in the order table, it leaves orphaned rows in the order and messages table. I want to delete these rows as well! The next thing is that this isn’t RESTful design. Instead of calling my method “delete,” let’s call it “destroy.” To fix the issue though, we need to use another method that has subtle difference than delete. Instead of delete, I should be using the destroy method. Why is that?

The delete method essentially deletes a row (or an array of rows) from the database. Destroy on the other hand allows for a few more options. First, it will check any callbacks such as before_delete, or any dependencies that we specify in our model. Next, it will keep the object that just got deleted in memory; this allows us to leave a message saying something like “Order #{order.id} has been deleted.” Lastly, and most importantly, it will also delete any child objects associated with that object!

Now my code in my controller looks like this.

def destroy
Order.destroy(params[:id])
end

Whenever I delete an order object, it will delete all child recipient and message rows in the database . This prevents any orphaned rows and allows for consistency in the database!

Home Projects Articles About Us 6 Ways to Run Shell Commands in Ruby

Often times we want to interact with the operating system or run shell commands from within Ruby. Ruby provides a number of ways for us to perform this task.
Exec

Kernel#exec (or simply exec) replaces the current process by running the given command For example:


$ irb
>> exec 'echo "hello $HOSTNAME"'
hello nate.local
$

Notice how exec replaces the irb process is with the echo command which then exits. Because the Ruby effectively ends this method has only limited use. The major drawback is that you have no knowledge of the success or failure of the command from your Ruby script.
System

The system command operates similarly but the system command runs in a subshell instead of replacing the current process. system gives us a little more information than exec in that it returns true if the command ran successfully and false otherwise.


$ irb
>> system 'echo "hello $HOSTNAME"'
hello nate.local
=> true
>> system 'false'
=> false
>> puts $?
256
=> nil
>>

system sets the global variable $? to the exit status of the process. Notice that we have the exit status of the false command (which always exits with a non-zero code). Checking the exit code gives us the opportunity to raise an exception or retry our command.
Note for Newbies: Unix commands typically exit with a status of 0 on success and non-zero otherwise.

System is great if all we want to know is “Was my command successful or not?” However, often times we want to capture the output of the command and then use that value in our program.
Backticks (`)

Backticks (also called “backquotes”) runs the command in a subshell and returns the standard output from that command.


$ irb
>> today = `date`
=> "Mon Mar 12 18:15:35 PDT 2007\n"
>> $?
=> #
>> $?.to_i
=> 0

This is probably the most commonly used and widely known method to run commands in a subshell. As you can see, this is very useful in that it returns the output of the command and then we can use it like any other string.

Notice that $? is not simply an integer of the return status but actually a Process::Status object. We have not only the exit status but also the process id. Process::Status#to_i gives us the exit status as an integer (and #to_s gives us the exit status as a string).

One consequence of using backticks is that we only get the standard output (stdout) of this command but we do not get the standard error (stderr). In this example we run a Perl script which outputs a string to stderr.


$ irb
>> warning = `perl -e "warn 'dust in the wind'"`
dust in the wind at -e line 1.
=> ""
>> puts warning

=> nil

Notice that the variable warning doesn’t get set! When we warn in Perl this is output on stderr which is not captured by backticks.
IO#popen

IO#popen is another way to run a command in a subprocess. popen gives you a bit more control in that the subprocess standard input and standard output are both connected to the IO object.


$ irb
>> IO.popen("date") { |f| puts f.gets }
Mon Mar 12 18:58:56 PDT 2007
=> nil

While IO#popen is nice, I typically use Open3#popen3 when I need this level of granularity.
Open3#popen3

The Ruby standard library includes the class Open3. It’s easy to use and returns stdin, stdout and stderr. In this example, lets use the interactive command dc. dc is reverse-polish calculator that reads from stdin. In this example we will push two numbers and an operator onto the stack. Then we use p to print out the result of the operator operating on the two numbers. Below we push on 5, 10 and + and get a response of 15\n to stdout.


$ irb
>> stdin, stdout, stderr = Open3.popen3('dc')
=> [#, #, #]
>> stdin.puts(5)
=> nil
>> stdin.puts(10)
=> nil
>> stdin.puts("+")
=> nil
>> stdin.puts("p")
=> nil
>> stdout.gets
=> "15\n"

Notice that with this command we not only read the output of the command but we also write to the stdin of the command. This allows us a great deal of flexibility in that we can interact with the command if needed.

popen3 will also give us the stderr if we need it.


# (irb continued...)
>> stdin.puts("asdfasdfasdfasdf")
=> nil
>> stderr.gets
=> "dc: stack empty\n"

However, there is a shortcoming with popen3 in ruby 1.8.5 in that it doesn’t return the proper exit status in $?.


$ irb
>> require "open3"
=> true
>> stdin, stdout, stderr = Open3.popen3('false')
=> [#, #, #]
>> $?
=> #
>> $?.to_i
=> 0

0? false is supposed to return a non-zero exit status! It is this shortcoming that brings us to Open4.
Open4#popen4

Open4#popen4 is a Ruby Gem put together by Ara Howard. It operates similarly to open3 except that we can get the exit status from the program. popen4 returns a process id for the subshell and we can get the exit status from that waiting on that process. (You will need to do a gem instal open4 to use this.)


$ irb
>> require "open4"
=> true
>> pid, stdin, stdout, stderr = Open4::popen4 "false"
=> [26327, #, #, #]
>> $?
=> nil
>> pid
=> 26327
>> ignored, status = Process::waitpid2 pid
=> [26327, #]
>> status.to_i
=> 256

A nice feature is that you can call popen4 as a block and it will automatically wait for the return status.


$ irb
>> require "open4"
=> true
>> status = Open4::popen4("false") do |pid, stdin, stdout, stderr|
?> puts "PID #{pid}"
>> end
PID 26598
=> #
>> puts status
256
=> nil