Using serializable hash

organization.rb

class Organization
  include ActiveModel::Serialization

  has_many :grants, as: :grants_made
  has_many :grants, as: :grants_received

  def grants_received_total
    grants_received.map(&:amount).inject(0, :+) # or .sum if you're all into ActiveSupport
  end
  
  def grants_made_total
    grants_made.map(&:amount).inject(0, :+)
  end

  def serializable_hash(options={})
    defaults = {
      methods: [:grants_received_total, :grants_made_total]
    }
    
    super options.merge(defaults)
  end

end