Validation failures when modifying collections

[elliot-nelson]

In our API, one of the primary models has several sub-model collections. These sub models all have validations, and the primary model also validates various aspects of the relationship between the collections. (For example: you can't save the Order if there isn't at least one Item. You can't have more Items than item_limit. Etc.)

When creating a record using representers, everything goes smoothly. However, when updating a record, these validations don't really work: even when using the find_or_instantiate strategy, it seems like the core call (somewhere along the line) is:

order.items = [new array of items]

Unfortunately, on an existing record, this is an instant call-to-action by Rails -- it runs off and inserts and deletes records, all before save is ever called on the parent object.

This behavior by Rails is one reason all our web forms use nested_attributes_for: the web forms are submitting the "items_attributes" collection, which means Rails doesn't attempt any database modifications until all the models are validated.

I saw another ticket (https://github.com/apotonick/roar-rails/issues/81) where you mentioned that you shouldn't need to use nested_attributes with roar. Do you have any patterns that would allow roar to behave the way way nested_attributes does? (That is: that will not make any database updates until all of the models being touched have been validated.)

[elliot-nelson]

As I play more with this, I think I can get pretty close to the behavior I'm looking for using the options parameters. Here's my current draft that is very close to what I want:

collection :sites, class: Site, extend: SiteRepresenter,
  instance: lambda {|hash, *| new_record? ? sites.build : (sites.select {|site| site.id == hash["id"]}.first || sites.build)},
  setter: lambda {|value, *| self.sites = value if new_record?}

It looks like what I probably want to do is continue to tune these blocks until the behavior is what I want, then I can encapsulate it as a Strategy and use it on all the collections in the object.

(Note the key difference between this and the existing find_or_initialize strategy is that I'm grabbing references to objects in memory, attached to the main object. That way the main object will validate and save the entire object tree.)

[apotonick]

First of all: HAPPY NEW YEAR! :fireworks:

Currently, Representable compiles the entire collection and then assigns it to the model. We will add another semantic to Representable, but I advise you to remove validations from your models and move them into Reform forms (or, contracts).

This works exactly like Roar/Representable but deserialises to object graph and wraps it with Reform form objects, validations can run without writing to the model/database, then, once you're happy, you can sync the state to the persistence layer.

Have you seen my Trailblazer book? This is all part of my Trailblazer architecture, you should have a look.