Overriding Equality of objects

In ruby the equality of the objects depend upon several things, which are

a) ==
b) hash
c) eql?

In normal cases whenever you run == method for string numbers etc it works perfectly fine, but if there are two identical objects of a class it will always return false.

Lets see an example:

class Car
  attr_accessor :name
  def initialize(name)
    @name = name
  end
end


p Car.new("swift") == Car.new("swift")
=> false

Thus we can see it gives the result false, even though both objects are identical. This is so because the hash of the objects are different and if every time you initialize an object it will return different hash value like this:

> Car.new("swift")
 => #<Car:0x000000022d85c8 @name="swift">
> Car.new("swift")
 => #<Car:0x000000022b4808 @name="swift">

Thus you can see the difference here, now suppose if you have a use case like this that if the attributes of the objects are identical then you need to show them as equal. To do that you need to override the == method in your class like this:

class Car
  attr_accessor :name
  def initialize(name)
    @name = name
  end

  def ==(other_obj)
    if self.name == other_obj.name
      return true
    else
      return false
    end
  end
end

 p Car.new("swift") == Car.new("swift")
 => true

 p Car.new("swift") == Car.new("fiat")
 => false

Here we have solve the problem of one operator but suppose we have an array of multiple objects and you want to perform uniq on them, then in that case it doesn't only depends upon the == operator. it is also dependent upon the eql? method and hash method. so we also need to override them too.

## before overriding

> [Car.new("swift"), Car.new("swift"), Car.new("swift")]
=> [#<Car:0x000000021c2288 @name="swift">, #<Car:0x000000021c2238
@name="swift">, #<Car:0x000000021c21e8 @name="swift">]

Now we will override eql? method and hash method

class Car
  attr_accessor :name
  def initialize(name)
    @name = name
  end

  ## Overriding == method
  def ==(other_obj)
    if self.name == other_obj.name
      return true
    else
      return false
    end
  end

  ##  Overriding eql method
  def eql?(other_obj)
        if self.name == other_obj.name
      return true
    else
      return false
    end
  end

  ## Overriding hash method
  ## Here we will just use the hash for the name attribute
  def hash
    self.name.hash
  end
end

 Now if you run the uniq on identical object array you will get only one element

> [Car.new("swift"), Car.new("swift"), Car.new("swift")].uniq
[#<Car:0x00000002059f18 @name="swift">]