Namespaced classes in Ruby

We can write namspaced classes in ruby in two ways.

Normal way we can wrap the class inside a module. Lets say the module name as ‘MyModule’.
And the constants we define inside this module are accessed as follows:


module MyModule
    CONST1 = 1
    class Myclass
       CONST2 = 2
       def name
          "This is my name"
       end

       def const_1
         CONST1
       end

       def const_2
          CONST2
       end
    end
end

p MyModule::Myclass.new.name
p MyModule::Myclass.new.const_1
p MyModule::Myclass.new.const_2

The other way of doing this is the short way of writing the class name with module name and two columns.
As you can see, the const_1 is accessed as prefixing the module name with two columns.

module MyModule
    CONST1 = 1
end

class MyModule::Myclass
    CONST2 = 2
    
    def name
       "This is my name"
    end
    
    def const_1
        MyModule::CONST1        
    end
    
    def const_2
        CONST2
    end
end

p MyModule::Myclass.new.name
p MyModule::Myclass.new.const_1
p MyModule::Myclass.new.const_2

There is an another way of doing this, that may looks strange to most of the people. Nested classes.

class Myclass
  def name
     "This is my name"
  end

  def my_class_2_name
     Myclass2.new.name
  end

  class Myclass2
    def name
       Myclass.new.name
    end
  end
end

> p Myclass.new.name
> "This is my name"
> p Myclass.new.my_class_2_name
> "This is my name"

The two printing works. So what is the use of these nested classes? Hmmm. It is just namespacing the second class and it tells, somehow it relates to first class even though there is no relation between these two classes.

> p Myclass2.new.name
> uninitialized constant Myclass2

We cannot access Myclass2 without specifying the namespace ( Myclass ) 

> p Myclass::Myclass2.new.name
> "This is my name"

Set up capistrano deployment for Ruby On Rails

STEP 1:
Install capistrano gem

group :test, :development do
  gem 'capistrano'
end

Install capistrano with rvm

gem 'rvm-capistrano'

STEP 2:
Prepare your Project for Capistrano
Capify your project. The following command initialise your project with Capistrano.

$ capify .

STEP 3:
Do proper modificatons in Capistrano Recipe (config/deploy.rb)
http://guides.beanstalkapp.com/deployments/deploy-with-capistrano.html

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

set :domain, "mydomain.in"
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"

STEP 4:
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)

STEP 5:
Deploy your project:

cap staging deploy

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

Suppose we have created a ‘BlogPost’ Model in Rails and it 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, Date.today - 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
  end

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
  end

How to create a migration file dynamically by meta programming in rails 4.0

If you want to create a migration file from a module written in lib file or somewhere from your ruby file and execute it, use the metaprogramming which can create a class or method dynamically. The following code snippet shows the methods we use and gives a better idea to create migration file dynamically.

def create_columns(tb_with_cols)
    add_columns = ""
    tb_name = tb_with_cols.keys.first
    columns = tb_with_cols.values.first
    columns.each { |c_name, c_type| add_columns << "\tadd_column(':#{tb_name}', :#{c_name}, :#{c_type})\n" }

    add_columns
 end

 def migration_file_content(tb_with_cols)
   cols = create_columns(tb_with_cols)
<<-RUBY
  class AddMissingColumnsToTable < ActiveRecord::Migration
     def change_table
    #{cols}
   end
 end
  RUBY
 end


 def write_content_to_file(path, content)
    File.open(path, 'w+') do |f|
      f.write(content)
    end
 end

Just call the method migration_file_content in your code. Pass the parameter tb_with_cols as a Hash, in which the key is the table_name and value is the columns that should be added to that table. Ex:

tb_with_cols = {:users => {:name => :string, :age => :integer, :address => :text} }
content = migration_file_content(tb_with_cols)
write_content_to_file("#{Rails.root}/db/migrations/', content)

After that call the method write_content_to_file with your new migration file path and the content from our migration_file_content method. 🙂

How to find a column type if table name and column name is given in Rails ActiveRecord

I found difficulty to find this. If we have table name and column name we can find this from ActiveRecord::Base.connection ‘column_for’ method.

Use the column_for method for finding the column type


ActiveRecord::Base.connection.column_for("table_name",  "column_name").type

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
     end
   def cb=(value)
     @cb = value
     end
   def cv
     @@cv
     end
   def cb
     @cb
     end
   end
 => nil

Then create an instance of this class a and b

Try accessing the class variable without initializing it.

a = Test.new
 => # 
 a.cv
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.

a.cv=45
 => 45 

a.cv
 => 45 

b = Test.new
 => # 
b.cv
 => 45 
a.cb = 34
 => 34 
a.cb
 => 34 
b.cb
 => nil 
b.cb = 90
 => 90 
b.cb
 => 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
    uri.to_s
end

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
    end
    uri.query = full_params.to_param
    uri.to_s
  end

How to implement a simple captcha in Rails using gem ‘simple_captcha’

Here it is, a simple captcha for Rails applications.

To set up, add the following line to your Gemfile

gem "galetahub-simple_captcha", :require => "simple_captcha"

Run rails generator

rails generate simple_captcha

Do db Migration

rake db:migrate -t

In your view file add

%= show_simple_captcha %

Add the following in yml file
en.yml:

en:
  simple_captcha:
    label: "Enter the above code"
    placeholder: "Enter here"

In controller just include captcha helpers module

class ApplicationController < ActionController::Base
  include SimpleCaptcha::ControllerHelpers
end

Use this code in the controller for the captcha validation

if simple_captcha_valid?
  do this
else
  do that
end

Details about pg gem error: Can’t find the ‘libpq-fe.h header when doing bundle in Rails

If you are getting errors like:

installing pdf-reader 1.3.3
Gem::Ext::BuildError: ERROR: Failed to build gem native extension.

/home/vagrant/.rvm/rubies/ruby-2.2.0/bin/ruby -r ./siteconf20150306-5919-1rr483p.rb extconf.rb
checking for pg_config... yes
Using config values from /usr/bin/pg_config
You need to install postgresql-server-dev-X.Y for building a server-side extension or libpq-dev for building a client-side application.
You need to install postgresql-server-dev-X.Y for building a server-side extension or libpq-dev for building a client-side application.
checking for libpq-fe.h... no
Can't find the 'libpq-fe.h header
*** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of necessary
libraries and/or headers. Check the mkmf.log file for more details. You may
need configuration options.

when trying to install pg gem,
you are just missing some dependancy library related to libpq. Install the following package


$ sudo apt-get install libpq-dev

then do


$ gem install pg -v 'version-no'

then do


bundle install