ActiveRecord Delete...?

Question

I'm just starting to learn ActiveRecord, and I am just trying out little things to figure out how everything works. I just tried the following code on the following sqlite3 database.

Ruby:

class Balances < ActiveRecord::Base
  def initialize
    @balance = 50
    update_attribute(:balance, @balance)
  end
  def withdraw amount
    update_attribute(:balance, @balance-amount)
  end
end

SQL:

CREATE TABLE balance(
    balance 50
);

When I write:

balance = Balances.new

I Get:

NoMethodError: undefined method `delete' for nil:NilClass
    from /Users/Solomon/.rvm/gems/ruby-1.9.3-p125/gems/activerecord-3.2.3/lib/active_record/attribute_methods/write.rb:28:in `write_attribute'
    from /Users/Solomon/.rvm/gems/ruby-1.9.3-p125/gems/activerecord-3.2.3/lib/active_record/attribute_methods/dirty.rb:67:in `write_attribute'
    from /Users/Solomon/.rvm/gems/ruby-1.9.3-p125/gems/activerecord-3.2.3/lib/active_record/attribute_methods/write.rb:14:in `balance='
    from /Users/Solomon/.rvm/gems/ruby-1.9.3-p125/gems/activerecord-3.2.3/lib/active_record/persistence.rb:180:in `update_attribute'

Why is this? Am I doing something wrong?

Larry K · Answer 1 · 2012-06-01T00:27:34.307

I notice several things:

The class name should be Balance (capitalized, singular). The table name in the database will be lower case, plural. Eg, balances
Don't define an initialize method for an ActiveRecord model. Instead use after_initialize callback. A post. Also, the Rails docs.

Added Also, the file name for the model should be balance.rb (lower case, singular)

Added some more You probably don't want to change the balance of the record back to 50 every time an instance of the record is initialized. -- That's what your example is currently doing. If you want to set the opening balance of new records in the database to be 50, then use the "before_create" callback.

Remember that ActiveRecord model classes are associated with, but different from, the records in the underlying database. For example, you can create an instance of an ActiveRecord model, and then not create a matching record in the database. -- The database record will only be created when and if you call the save or create methods.

ActiveRecord Delete...?

1 Answers1