Working with unsaved associations

Question

Let's say I have two models:

class Model1 < ActiveRecord::Base
  has_many :model2

  def save
    self.attr = <sth. complex involving the associated model2 instances>
    super
  end
end

class Model2 < ActiveRecord::Base
  belongs_to :model1
end

The statement in the overwritten save method will issue a complex query (using find [or alternatively named scopes]) to calculate some aggregate value for some of the associated Model2 instances. The problem is that when a new Model1 instance along with some Model2 instances, this query will not return anything on the first save after the objects have been created and will return old data (the previous generation) for all consecutive save operations.

Is there a way to use find on the non-persisted in-memory state?

Answer 1

Of course: it's called #select, or #inject, or any of the other -ect methods:

class Model1 < ActiveRecord::Base
  has_many :model2

  before_save :update_aggregate_value

  private
  def update_aggregate_value
    # returns sum of unit_value for all active model2 instances
    self.attr = model2.select(&:active?).map(&:unit_value).sum
  end
end

class Model2 < ActiveRecord::Base
  belongs_to :model1
end

Remember in this case that the DB will not be hit, except if the model2 instances hadn't been loaded yet. Also, notice I used a before_save callback, instead of overriding #save. This is "safer" in that you can return false and prevent the save from proceeding.