Good Rails Habits

Use only one dot.

@invoice.customer.address is bad. What if the app changes so that the customer has both a billing and a shipping address? Use the delegate method to wrap without clutter.

Ref: Rails AntiPatterns, P23

A model shouldn't know the implementation details of another model.

class Pet < ActiveRecord::Base
  has_many :toys

  def find_cute_toys
    self.toys.where(:cute => true)
  end

This isn't good because Pet shouldn't know that Toy has a binary 'cute' column. This should be moved to the Toy model. (This actually needs a better example because the find_cute_toys method makes more sense to be on the Toy model as a scope, even without the implementation detail issue.)

Ref: Rails AntiPatterns, P52

Don't forget that div_for is handy

No need to set custom id's when you can do <%= div_for %>

Don't forget to use an index to keep model data (such as emails or usernames) unique

class AddIndexToUsersEmail < ActiveRecord::Migration
  def change
    add_index :users, :email, unique: true
  end
end

Speed up Factory Girl tests by using unsecure passwords in test environment

config/environments/test.rb
ActiveModel::SecurePassword.min_cost = true