Custom builder method for an ActiveRecord association (Logic localised to has_many's class)

activerecord_association_custom_builder

class Line << ActiveRecord::Base
  belongs_to :group
  belongs_to :product
  
  def build_for_product(product_id)
    product = Product.find(product_id)
    line = Line.new(
      :product => product,
      :line_type => 'stock',
      :quantity => 1,
      :order_unit => product.stock_unit,
      :unit_price_cents => product.sell_price_cents,
      :price_basis => 'extax'
    )
    line
  end
end

class Group << ActiveRecord::Base
  has_many :lines do
    def build_for_product(product_id)
      # Build a line using a custom build method 'build_for_product()' class method on Line
      # IE @group.lines.build_for_product(123)
      # line can either be saved via line, or via group.save
      self.build(Line.build_for_product(product_id).attributes)
    end
  end
end

## Example
group = Group.first
line = group.lines.build_for_product(13)

line # <#Line id: nil, group_id: 5, line_type: "stock", state: "dispatching", product_id: 13, quantity: 1, order_unit: nil, unit_price_cents: 0, currency: "AUD", price_basis: nil, created_at: "2015-03-26 02:10:16", updated_at: "2015-03-26 02:10:17">

group.save OR line.save