Active Record get all children of children and so on

I am using MongoDb as database.

I want all the children of children and so on. Lets Suppose

• A has B & C children
• B has D & E children
• D has F & G children

So when I query for children node A. I get all the children as output such as B C D E F G

 C = Customer.find_by(:id => "SOME_ID")
 C.children #list all children upto one level

So can anyone suggest me the way, to get recursive children.

Customer model

class Customer

  include Mongoid::Document
  field :email, type: String
  field :referral_id, type: String
  belongs_to :parent, class_name: 'Customer',foreign_key: "referral_id", optional: true
  has_many :children, :class_name => 'Customer', :foreign_key => "referral_id"

end

Can anybody please help me out. Or suggest a way to accomplish this.