The Rails Drop

In class 1 we defined the instance variables and know how difficult to accessing it. We managed to access it via calling methods that initializing it.
So we can simplify it by defining access methods.

class Test
 def initialize
  @one = 1
 end 
 def get_one
   @one
 end 
end

The ‘get_one’ method is the reader method for reading the instance variable value.

Ruby has shortcut for this.

class Test
 attr_reader :one
 def initialize
  @one = 1
 end 
 
end

These accessor methods are identical to the methods we wrote by hand earlier.

Sometimes we need to modify the value of these variables from outside. how we do that?

At first we can do this by hand:

class Test
 def initialize
  @one = 1
 end 
 def one=(new_value)
   @one = new_value
 end 
end

The above code allows us to call ‘equal to’ sign on the objects method. This is the setter method. We can set a value to @one like:

t = Test.new
t.one = 3
t.one
=> 3

Ruby also provides a shortcut for this. It is: ‘attr_writer’, so rewrite the above code:

class Test
 attr_writer :one
 def initialize
  @one = 1
 end 
end

Most of the cases we needed ‘attr_reader’ and ‘attr_writer’. So the code becomes:

class Test
 attr_reader :one
 attr_writer :one

 def initialize
  @one = 1
 end 
end

We can again simplify the above code!

class Test
 attr_accessor :one

 def initialize
  @one = 1
 end 
end

How is it? Nice. is n’t it?

We read so many documents about ruby but do you think about some facts that we really don’t know. See below.

1. Instance variables are accessible across the class for that instance

Example:

class Test2
 def one
  @one = 1
  @two
 end
 def two
  @two = 2
  @one
 end
end

Here

t = Test.new
=> 
t.one is what ?
t.two is what ?

Can you guess?

t.one is nil why? t.one will return @two, but it is not initialized yet. We have to call t.two for intitializing it (the variable gets a value).

t.two is what?
Ans: 1
Why? because we already initialized @one by calling t.one above. If not it will also return nil.

So what is the purpose of instance variable if we are not getting its value when initializing the Class (say t = Test.new)

Here is the importance of ‘initialize’ method. Lets change the method named ‘two’ to ‘initialize’

class Test3
 def one
  @one = 1
  @two
 end
 def initialize
  @two = 2
  @one
 end
end

then you try:

t = Test.new
=>    ### See here already declared the variable @two

t.one
=> 2

Nice. So we can access that instance variable anywhere in the class. How is that? 🙂

How to send gmail calendar events to user emails in Rails? Lets see how we can do this. Here we use Rails 4.2 and Ruby 2.2.1 versions.

For this we can use the best available package the ‘icalnder’ gem.

Icalender Github

In Gemfile add:

gem 'icalendar'

We have a user_mailer.rb for sending emails to User. And suppose we are sending emails to manager (User type 1) and a participant (User type 2) when a conversation is scheduled between them.

Create a method as below:

in user_mailer.rb

   def conversation_scheduled(conversation_id, opts={})
      @conversation = Conversation.find_by_id(conversation_id)
      @manager = @conversation.try(:manager)
      @participant = @conversation.try(:participant)
      @base_url = opts['base_url']

      if opts[:send_to] == 'manager'
        cal = calender_event(@conversation, @manager, "Conversation is scheduled with #{@participant.full_name}", "You have scheduled a conversation with Mr/Mrs #{@participant.full_name} on #{@conversation.starting_time}")  
        attachments['conversation_sheduled.ics'] = { :mime_type => 'text/calendar',
                              :content => cal.to_ical }

        mail(to: @manager.try(:email),
             subject: "Conversation is scheduled with #{@participant.full_name}",
             template_name: 'scheduled_manager')
      elsif opts[:send_to] == 'participant'
        cal = calender_event(@conversation, @manager, "Conversation is scheduled with #{@manager.full_name}", "A conversation is scheduled with Mr/Mrs #{@manager.full_name} on #{@conversation.starting_time}")  
        attachments['conversation_sheduled.ics'] = { :mime_type => 'text/calendar',
                              :content => cal.to_ical }

        mail(to: @participant.try(:email),
             subject: "Conversation is scheduled with #{@manager.full_name}",
             template_name: 'scheduled_participant')
      end
    end

private 

def calender_event(conversation, manager, summary, description)
      cal = Icalendar::Calendar.new
      cal.event do |e|
        e.dtstart     = Icalendar::Values::Date.new(conversation.startime.to_date.strftime("%Y%m%dT%H%M%S%Z"))
        e.dtend       = Icalendar::Values::Date.new(conversation.end_time.strftime("%Y%m%dT%H%M%S%Z"))
        e.summary     = summary
        e.description = description
        e.ip_class    = "PRIVATE"
        e.organizer = Icalendar::Values::CalAddress.new("mailto:#{manager.try(:email)}", cn: manager.try(:full_name))
      end
      cal
    end

All we need to do is send an attachment to the email setting the proper mime type to ‘text/calendar’ and the content by calling the ‘to_ical’ method on the event and it returns the calender object.

With proper email templates I do receive the emails with Calendar events and I can add this event to my google calendar and I can update the event details etc.

Hi guys, how to validate a Rails belongs_to association in most effective way? Did you ever think about that? I am asking this question because there is something that most of the people will not notice. I can explain.

Suppose we have 2 Models ‘User’ and ‘Group’

Condition:
Every ‘User’ must belongs to one of the ‘Group’.

So how we validate this? A simple way of doing this is:

Validate foreign key

class User
  belongs_to group
  validates :group_id, presence: true
end

What this code actually does? Lets find out.

> Group.last.id # => 80
> User.new(group_id: 100).valid? # => true
> Group.exists?(100) # => nil

Oops… Is that true? Not at all. So rails make us fool? Ha. The presence validator will only check a foreign key is provided to it. That’s it. And it will not check the existence of that record. So what we can do in this case? Lets check the other way of doing this.

Validate associated record

class User
  belongs_to group
  validates :group, presence: true
end

Here what the validator does is it checks the record exists or not. If record not exists it returns false for valid? method.

> Group.last.id # => 80
> user = User.new(group_id: 100)
> user.valid? # => false
> Group.exists?(100) # => nil
> user.errors.full_messages # => ["Group can't be blank"]

Wahh..great! But wait, think.. What it actually does in the background? Whenever a user record goes through the .valid? call, rails fire a database query to find this record exists or not. Is that really good? In one way. In other way it’s bad. It affects performance.

But we have to consider database integrity. So the second approach wins even though it is firing an extra query in the background. Cheers!