In normal rails application contain one database,but if we want rails application having more than one database that is multiple data base.   we can achieve this using multiple way.one of the way is i am showing here.it’s just 3 steps. Lets take an e.g Project has many milestone and milestone has many task In normal scenario model looks like following way class Project < ActiveRecord::Base has_many :milestones end   class Milestone < ActiveRecord::Base has_many :tasks belongs_to :project end   …