Create bootable usb drive of OSX from Mac OS

You can read the following document to get the clear picture:

Use mac’s createinstallmedia command. The format is given below.

$ sudo /Applications/Install\ macOS\ --volume |YOUR-USB-DRIVE-PATH-HERE| --applicationpath |DOT-APP-FILE-MACOS|

In My case the command will be as follows:

$ sudo /Applications/Install\ macOS\ --volume /Volumes/ABHI\'S/ --applicationpath /Applications/Install\ macOS\

#Rails 4.2 #Ruby2.2 How to find association class and other info from an object and its association name

When I was doing something, I encountered one situation like: I need the association class of an object. I have the object and its association name as input. How can I find the association class?

Suppose we have Student class that belongs to a school

class School
  has_many students

class Student
  belongs_to :school

and so many other relations like this in my project.

So I have

s = Student.last
:school symbol 

in my hand

I can use and

But what if the school is blank? The result is ‘NilClass’ From the above code.

Basically for has_many associations we gets the class name as


because recently in new versions rails changed the Array of objects as associations to its own ‘CollectionProxy’.

So we can use ‘ActiveRecord::Reflection::ClassMethods’ for finding all the association info.

This Rails module is so useful to find all the association related information.

In the above situation we can use ‘reflect_on_association’ method for finding association reflection info. And it returns ‘ActiveRecord::Reflection’ Object.

> s.class.reflect_on_association(:school)
=> ##<ActiveRecord::StatementCache:0x000000037a5c08 
@query_builder=#, @bind_map=#<ActiveRecord::StatementCache::BindMap:0x000000037a7238 
@indexes=[0], @bind_values=[[#
, #
]]>>}, @scope_lock=#
, @class_name="Topic", @foreign_key="school_id">

PostgreSQL 9.3 : Installation on ubuntu 14.04

Hi guys, I just started installing postgres on my ubuntu VM. I referred some docs, and followed this one:

Its pretty much explained in this page. But just explaining here the important things.

You can install postgres by ubuntu’s own apt packaging system. Update local apt repository, if u want.

$ sudo apt-get update
$ sudo apt-get install postgresql postgresql-contrib

Postgres uses role based access for the unix users. After the installation a default role called ‘postgres’ will be created. You can login to postgres account and start using or creating new roles with Postgres.

Sign in as postgres user

$ sudo -i -u postgres

Access the postgres console by

$ psql

But i cannot enter into the console and I got the following error:

postgres@8930a29k5d05:/home/rails/my_project$ psql
psql: could not connect to server: No such file or directory
        Is the server running locally and accepting
        connections on Unix domain socket "/var/run/postgresql/.s.PGSQL.5432"?

What could be the reason for this error?

So just gone through Postgres doc ( You can see the same error under the section 17.3.2. Client Connection Problems. But the solution is not mentioned.

Original Reason: PostgreSQL Server was not running after the installation.

I tried to reboot and thought, via init script the server will run automatically. That also not happened. So I understood that something prevents postgres from running the server. What is it?

Just check your postgres server is running or not

$ sudo -aux | grep post
postgres@8930a29k5d05:/home/rails/my_project$ ps -aux | grep post
root       136  0.0  0.2  47124  3056 ?        S    06:10   0:00 sudo -u postgres -s
postgres   137  0.0  0.3  18164  3220 ?        S    06:10   0:00 /bin/bash
postgres   140  0.0  0.2  15572  2192 ?        R+   06:10   0:00 ps -aux
postgres   141  0.0  0.0   4892   336 ?        R+   06:10   0:00 grep post

The server is not running.

Run the server manually by

root@8930a29k5d05:/home/rails/my_project#  /etc/init.d/postgresql start
 * Starting PostgreSQL 9.3 database server
                                                                                                                                                         [ OK ] 
root@8930a29k5d05:/home/rails/my_project# ps aux | grep post
postgres   158  0.1  2.0 244928 20752 ?        S    06:28   0:00 /usr/lib/postgresql/9.3/bin/postgres -D /var/lib/postgresql/9.3/main -c config_file=/etc/postgresql/9.3/main/postgresql.conf
postgres   160  0.0  0.3 244928  3272 ?        Ss   06:28   0:00 postgres: checkpointer process

postgres   161  0.0  0.4 244928  4176 ?        Ss   06:28   0:00 postgres: writer process

postgres   162  0.0  0.3 244928  3272 ?        Ss   06:28   0:00 postgres: wal writer process

postgres   163  0.0  0.5 245652  6000 ?        Ss   06:28   0:00 postgres: autovacuum launcher process

postgres   164  0.0  0.3 100604  3336 ?        Ss   06:28   0:00 postgres: stats collector process

root       178  0.0  0.0   8868   884 ?        S+   06:28   0:00 grep --color=auto post

Now it is working. If still not works, then try to reconfigure your locales as mentioned here

$ dpkg-reconfigure locales

It is strange that, after installing such a popular db software, it doesn’t provide any information regarding the failure of its own server. It should give the developers some clue so that they can save their precious time.

The reason of this failure, that I concluded is
1. After installation we have to run the server manually
2. I tried resetting the locales (So if no locales set in the machine may prevented the postgres from starting automatically?)

Set up capistrano deployment for Ruby On Rails

Install capistrano gem

group :test, :development do
  gem 'capistrano'

Install capistrano with rvm

gem 'rvm-capistrano'

Prepare your Project for Capistrano
Capify your project. The following command initialise your project with Capistrano.

$ capify .

Do proper modificatons in Capistrano Recipe (config/deploy.rb)

Lets do the deployment for staging environment.
Create a ruby file under config/deploy/ folder named staging.rb
Copy the following content

set :domain, ""
role :app, domain
role :web, domain
role :db, domain, :primary => true
role :resque_worker, domain   # if you are using workers in your project, set role for them if needed
role :resque_scheduler, domain # if you are using workers in your project

set :deploy_to, "/home/my_deploy_path/"  # the deployment directory
set :environment, "staging"
set :rails_env, "staging"
set :branch, "staging"
set :previous_environment, "develop"

Setup capistrano in deployment server

$ cap staging deploy:setup

This will Create folder structure that capistrano uses in the process.

Make sure that everything is set up correctly on the server by the command

$ cap staging deploy:check

Now you can see a message like:
“You appear to have all necessary dependencies installed”

Create shared/config folder in your deploy_to path
and copy database.yml and other config files as you written in the symlink_shared task in cap recipie (if any)

Deploy your project:

cap staging deploy

How to categorise a blog posts data by month in Ruby On Rails

Suppose we created a ‘BlogPost’ Model in Rails and we have the following fields in a blog post:

title – title of the blog post
posted_on – date posted
permalink – a permanent link of each blog post (act as a primary key)
publish – a boolean field which decides the post need to show or not

Lets write a method in ‘BlogPost’ Model to get a recent list of posts.
Pass a ‘months_old’ parameter to determine how much months old posts we wanted to list.
Just select the required columns to show the details of the post (by ‘:select => ‘). And Group each post by posted month.

  def self.get_recent_months_post(months_old)
    @blog_posts = where("publish = ? AND posted_on > ?", true, - months_old.months).all(:select => "title, posted_on, permalink", :order => "posted_on DESC")
    (@blog_posts.group_by { |t| t.posted_on.beginning_of_month }).sort.reverse

We successfully written the method above. Now lets write a method to get the archives (old posts).

  def self.get_archives(old)
    @blog_posts = where("publish = ? AND posted_on  "title, posted_on, permalink", :order => "posted_on DESC")
    (@blog_posts.group_by { |t| t.posted_on.beginning_of_month }).sort.reverse

An Example for the difference between class and instance variable in Ruby

Create a ruby class called Test

class Test
   def cv=(value)
     @@cv = value
   def cb=(value)
     @cb = value
   def cv
   def cb
 => nil 

Then create an instance of this class a and b

Try accessing the class variable without initializing it.

a =
 => #
NameError: uninitialized class variable @@cv in Test
    from (irb):9:in `cv'
    from (irb):16

Initialize the class variable with some data. And add values to the instance variable of a and b.
 => 45
 => 45 
b =
 => #
 => 45 
a.cb = 34
 => 34 
 => 34 
 => nil 
b.cb = 90
 => 90 
 => 90 

Rails way of creating a new full url with new parameters after removing old parameters from a url

If we have some keys and values and we just need to create a url with this data just pass the url and hash as a parameter to the following method.

def generate_url_with_params(url, params = {})
    uri = URI(url)
    uri.query = params.to_query

If you want to get rid of the parameters from a url use the following method.

  def generate_url_without_params(url, params = {})
    uri = URI(url)
    full_params = Rack::Utils.parse_query uri.query
    params.each do |key, val|
      full_params.delete key
    uri.query = full_params.to_param