Hi,
QUESTION: How can establishing validation that spans multiple models be
achieved in Rails? That is in such a fashion that it is not possible for a
developer to break the validation via using any of the public model methods
(e.g. update_attribute, save, create etc).
EXAMPLE:
* Concept: MAGAZINE can contain multiple ARTICLES, and an ARTICLE can be
associated with multiple MAGAZINE (i.e. many to many). Cost of Magazine =
Sum(Cost of Articles)
* Tables:
(a) magazines (has "cost" field)
(b) articles_magazines (to map the many-to-many)
(c) articles (has "total_cost" field)
BUSINESS RULE to be implemented: Not possible for a database update that
would end up with a Magazine's "total_cost" not being equal to the
Sum(associated articles "cost"s)
ISSUES / QUESTIONS:
(1) Assume would not try to implement rules at database constraint level???
(2) Use of Model "before_create" - but I'm assuming here if the Article is
generated (Article.new), and then validation occurs, the code has NOT yet
got to the bit where it updates the Magazine?
(3) Use of "after_create" - Add a check for both Magazine and Article
perhaps here, noting the database record has been created by transaction NOT
finalised yet. So would the following be the best way:
-----example-------
class Magazine < ActiveRecord::Base
after_save :business_rule_validation
def business_rule_validation
sum_of_articles = << INSERT code that calculates SUM of Articles costs
for all articles that are associated with the Magazine >>
errors.add_to_base("business rules fail") if self.total_cost !=
sum_of_articles
end
end
class Article < ActiveRecord::Base
<< Add Same Concept as per Magazine >>
end
-----example-------
BUT wouldn't this fail, as it assumes the Article create/update/delete and
the Magazine create/update/delete is in the SAME transaction no??? Does
this mean you really have to create an overarching facade that handles
creates/updates/deletes for Article/Magazines and somehow hide the normal
per model save/update/delete???
--
Greg
http://blog.gregnet.org/
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