Changes, changed? and <attribute>_was explained

rails_tip/2016-03-31-changes_and_changed.md

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+---
+title: Check status of ActiveRecord Object
+tip-number: 30
+tip-username: Surender
+tip-username-profile: https://github.com/harrysuren
+tip-description: We can check the status of an ```ActiveRecord``` Object and its variables by using these methods.
+
+---
+
+
+ActiveRecord keeps a sizable amount of information about an object while it undergoes updates. A hash, accessible via the changes method, acts a central location for these updates.
+
+**```changes```**
+
+Each key in the changes hash maps to a record’s attribute. The value of each key is an Array with two elements: what the attribute value used to be, and what it is now:
+
+```ruby
+  order = Order.find(1)
+  order.user_id
+  # => 10
+  order.user_id = 11
+  order.changes
+  # => {"user_id"=>[10, 11]}
+```
+
+**a. ```changed?```**
+
+A simple boolean method changed? is available to determine if anything on the object is different since its retrieval.
+
+```ruby
+  order = Order.find(1)
+  order.user_id = 11
+  order.changed?
+  # => true
+```
+
+However, if the same value is set on a model, regardless of the value’s object_id, changed? returns false:
+
+```ruby
+  order = Order.find(1)
+  order.user_id
+  # => 1
+  order.user_id = 1
+  order.changed?
+  # => false
+```
+
+Two “different” strings were set to book.title but since they resolve to the same characters, the object is not changed?.
+
+When using PostgreSQL, the ```changed?``` method can save unnecessary ```begin``` and ```commit``` transaction calls for objects that have no changes:
+
+```ruby
+  class OrdersController < ApplicationController
+    def update
+      @order = Order.find(order_params[:id])
+      @order.assign_attributes(order_params)
+      @order.save! if @order.changed?
+    end
+  end
+```
+
+**b. ```<attribute>_was```**
+
+Accessing the same ```changes``` hash as the ```changed?``` method, ```<attribute>_was``` returns the value of an attribute before it was reassigned:
+
+```ruby
+  order = Order.find(1)
+  order.status
+  # => 'waiting'
+  order.status = 'delivered'
+  # => 'delivered'
+  review.status_was
+  # => 'waiting'
+```