Rails - attr_accessor :- 读取属性值而不是覆盖它的函数

Question

I have a model Customer that accepts a virtual attribute opening_balance . Code looks something like this:

model Customer < ApplicationRecord
  attr_accessor :opening_balance

  has_one :ledger_account

  after_create :open_ledger_account

  def opening_balance
    ledger_account.opening_balance if ledger_account.present?
  end

  private

  def open_ledger_account
    create_ledger_account!(opening_balance: self.opening_balance)
  end

But the issue here is self.opening_balance is calling the method defined in the class not the value stored in attr_accessor's opening_balance attribute.

I tried one more solution:

def open_ledger_account
  create_ledger_account!(opening_balance: self.read_attribute(:opening_balance))
end

But this also doesn't work.

How to read the value stored in the actual attribute? Any help would be appreciated. I am using rails 5.1.

Thanks!

Answer 1

attr_accessor defines a instance variable and a method to access it (read/write).

So the easy way is to write:

def open_ledger_account
  create_ledger_account!(opening_balance: @opening_balance)
end

The read_attribute would only work if opening_balance was an attribute in the database on Customer .

Answer 2

First you have to understand that attr_accessor does not define instance variables. It just creates setter and getter methods. What attr_accessor :name does is:

class Person
  def name
    @name
  end

  def name=(value)
    @name = value
  end
end

Now you can access the instance variable from the outside:

p = Person.new
p.name = 'Jane'
puts p.name

And you can also access the instance variable from the inside by using the getter method instead of @name :

class Person
  attr_accessor :name
  def hello
    "hello my name is: #{name}"
  end
end

attr_accessor does not "define" a instance variable. There is no declaration of members/attributes in Ruby like in for example Java. An instance variables is declared when it is first set. Accessing an instance variable that has not been assigned a value returns nil.

So whats happing here:

class Customer < ApplicationRecord
  attr_accessor :opening_balance
  # ...
  def opening_balance
    ledger_account.opening_balance if ledger_account.present?
  end
end

Is that you are overwriting the getter created by attr_accessor . If you want to access the instance variable itself just use @opening_balance .

However...

You should just use delegate instead:

class Customer < ApplicationRecord
  has_one :ledger_account
  delegate :opening_balance, to: :ledger_account
end