What happens when you run Rails migration?

Rails migration allows the creation of the table and provides the functionality that can be performed on a table with the following commands:

create_table(name, options)
drop_table(name)
rename_table(old_name, new_name)
add_column(table_name, column_name, type, options)
rename_column(table_name, column_name, new_column_name)
change_column(table_name, column_name, type, options)
remove_column(table_name, column_name)
add_index(table_name, column_name, index_type)
remove_index(table_name, column_name)

-Rails Migration also allows the use of pre-defined data type in the application as it supports all the data types. The data types consist of string, integer, float, etc.

-Rails Migration allows the users to use the valid column options like limit (:limit=> "50"), default (:default => "hello"), and null (:null => false implies NOT NULL)

What is the difference between Symbol and String in Ruby?

The symbol in Ruby on rails act the same way as the string but the difference is in their behaviors that are opposite to each other.

The difference remains in the object_id, memory and process time for both of them when used together at one time.

Strings are considered as mutable objects. Whereas, symbols, belongs to the category of immutable.

Strings objects are mutable so that it takes only the assignments to change the object information. Whereas, information of, immutable objects gets overwritten.

String objects are written like
irb(main):001:0> "string object apple".object_id #=>70278982314860
or
irb(main):002:0> "string object apple".to_sym.object_id #=> 456148

Symbols are used to show the values for the actions like equality or non-equality to test the symbols faster then the string values.

Difference between Argument and Parameter in ruby on rails?

A parameter represents a value that the method expects you to pass when you call it.
An argument represents the value you pass to a method parameter when you call the method. The calling code supplies the arguments when it calls the method.

In simple words, parameters appear in method definitions; arguments appear in method calls.

For example, in below method, variables param1 and param2 are the parameters

def foo_method(param1, param2):
    .......
end
while calling the method, arg1 and arg2 are the arguments
 
foo_method(arg1, arg2)

What are filters? and how many types of filters are there in ruby?

Filters are methods that are run before, after or "around" a controller action.

Filters are inherited, so if you set a filter on ApplicationController, it will be run on every controller in your application.

Filter can take one of three forms: method reference (symbol), external class, or inline method (proc).

          after_filter
    append_after_filter
    append_around_filter
    append_before_filter
    around_filter
    before_filter
    filter_chain
    prepend_after_filter
    prepend_around_filter
    prepend_before_filter
    skip_after_filter
    skip_before_filter
    skip_filter

What is the difference between save and save! in Rails?

Save! performs all validations and callbacks. If any validation returns false, save! throws an error and cancels the save.

Save does not throw any error in the case above, but cancels the save. Also, the methods for validations can be bypassed.

What is the difference between delete and destroy in Rails?

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 “ 'Object_name' has been deleted.” Lastly, and most importantly, it will also delete any child objects associated with that object!

Case equality operator === in ruby

To enable developers to write neat, compact code, Many classes take advantage of implementing their own versions of '==='. The '===’ operator is known as the case equality operator. The '===’ operator is used when testing cases in case statements.

def test_object number 
  case number
    when Fixnum
      case number 
        when 1  
          puts "It's One"
        when 2..10  
          puts "Between two and ten"
        else
          puts "-- #{number} -- is not between one and ten"
      end
    when String 
      puts "You passed a string"
    else "I have no idea what to do with -- #{number} --"
  end
end

for each 'when' statement the '===' operator is called on the when argument (1,2..10,String) and the case argument (number) is used for the comparison. For line 5 the comparison done shall be '1 === number'. For most classes ‘===’ is just an alias to ‘==’ however some classes redefine ‘===’ to allow for better functionality in case statements. In Ruby a Proc is a block of executable code, an anonymous function which can be passed around as if it were data. Like ‘===’, a Proc can map a number of arguments to either a true or false value.

is_weekday = lambda {|day_of_week, time| time.wday == day_of_week}.curry  
      
    sunday    = is_weekday[0]  
    monday    = is_weekday[1]  
    tuesday   = is_weekday[2]  
    wednesday = is_weekday[3]  
    thursday  = is_weekday[4]  
    friday    = is_weekday[5]  
    saturday  = is_weekday[6]  
      
    case Time.now  
    when sunday   
      puts "Day of rest"  
    when monday, tuesday, wednesday, thursday, friday  
      puts "Work"  
    when saturday  
      puts "Happy"  
    end  

Simple Factorial lambda function in Ruby

factorial = lambda do |n|
  n <= 1 ? 1 : (n * factorial.call(n - 1))
end

factorial.call 3