Rails - Validate Presence Of Association?

You can use validates_presence_of http://apidock.com/rails/ActiveModel/Validations/ClassMethods/validates_presence_of

class A < ActiveRecord::Base
  has_many :bs
  validates_presence_of :bs
end

or just validates http://apidock.com/rails/ActiveModel/Validations/ClassMethods/validates

class A < ActiveRecord::Base
  has_many :bs
  validates :bs, :presence => true
end

But there is a bug with it if you will use accepts_nested_attributes_for with :allow_destroy => true: Nested models and parent validation. In this topic you can find solution.