Legacy AR database -- polymorphic association

[blelump]

Hello!

Since ROM API (and docs!) is going better and better with each release, we'd like to taste it for real. However, carrying a legacy AR DB needs some solution for 'polymorphic association' antipattern I haven't found. I've seen this and this, but none of them offer complete answer, at least for me. Let's continue with the code snippet given here and say that I have Snapshot and want to get the Questions, based on the parent_type attribute. In particular:

        snapshots.by_id(id)
          .combine(one: polymorphic_association_name...?)
          .one!

So the result is an aggregate of snapshot and whatever is given within the polymorphic association. In AR world, the aggregate's child is resolved based upon the parent_type column, but how to specify relation name based on that column value?

I'm not expecting any one-line solution, I'd be happy with any 😄 .

[solnic]

Like that: https://gist.github.com/solnic/7df076239cfe79f6459b3f6ea6c7d741

[blelump]

Hi Piotr!

Thanks for the gist ! How would you query from tag_repo for whatever is given for particular record? For instance, having insterted book_tag, how to get the aggregate of tag (the aggregate root) and book? In short, the opposite direction from your example (so you get: ROM::Struct[Tag] id=1 ... book=[#<ROM::Struct[Book] id=1 ...). Sorry if I haven't been more concrete previously.

[solnic]

@blelump updated the gist with tags => {song,book} associations too

[blelump]

@solnic , thanks for update ! However, I'll dig deeper ;-) . What if you dont know where particular tag is bound to? Say you want you want exactly ROM::Struct[Tag] id=1 ... book=[#<ROM::Struct[Book] id=1 ... , but you don't know the tag is associated to book. In essence, you don't know what's the value of taggable_type. I imagine:

puts tag_repo.aggregate(:taggable).inspect
=> `ROM::Struct[Tag] id=1 ... taggable=[#<ROM::Struct[Book] id=1 ...`

That may be difficult and I imagine you'd achieve the above entity with two separated queries and then merge the result with Transproc features. Is there any other solution (by just utilizing ROM)?

[solnic]

@blelump yes it's possible but requires custom relation composition, as associations don't support overridding query logic (you can only extend them via :view option, but it's not enough in this case). I'll prep another gist later today or tomorrow.

[solnic]

I marked it as a feature for v2.0.0 milestone. We don't want this to be a first class feature, but it can be a plugin that people will be able to turn on when they need to connect to a legacy, post-AR database schema.

I imagine something like this could be done:

class Songs < ROM::Relation[:sql]
  schema(infer: true) do
    use :ar_polymorphic_associations

    associations do
      has_many :taggings, foreign_key: :taggable_id
      has_many :tags, as: :taggable, through: :taggings
    end
  end
end

class Tags < ROM::Relation[:sql]
  schema(infer: true) do
    use :ar_polymorphic_associations

    associations do
      belongs_to :taggable, polymorphic: true
    end
  end
end

[flash-gordon]

schema plugins? I started to think they'll be useful for auto-indexing and other type-manipulation things

[blelump]

@solnic , thanks in advance for another gist ! I completetely understand you don't want it as a basic feature. IMO it doesn't have to be a plugin either if the overall complexity of the acceptable solution is a few extra LoC. The API should be clear and straightforward, but on the other hand there're other places for sure that need one/your's attention (& improvements).

[solnic]

@blelump ayee, hence "help-wanted" label 🤣

@flash-gordon yeah I think we need schema plugins, as schemas are separate, standalone objects, so extending them through relation plugins would be awkward :)

[flash-gordon]

@solnic exactly!

[blelump]

Hello @solnic , sorry for the remainder, any progress on the taggable ? 🙄

[solnic]

No, sorry. Had unexpected errands to run and zero time for oss today. I'll try to tackle it tomorrow. Thanks, Piotr

…

On 8 Mar 2017, 5:44 PM +0100, Michał Pietrus ***@***.***>, wrote: Hello @solnic (https://github.com/solnic) , sorry for the remainder, any progress on the taggable ? 🙄 — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub (#73 (comment)), or mute the thread (https://github.com/notifications/unsubscribe-auth/AAAEKkjsZbtXOdnrZPOaIKIxGoDc8w_Wks5rjtsFgaJpZM4MUY_s).

[blelump]

Hi @solnic ,

I've tried to investigate it further and as I understand, at least from the public API, I need to provide another relation in order to 'combine' the current relation with something. However, I imagine, to make it working, I need to 'edit' current relation somehow (without another relation since it's not known yet -- I mean whether it's a book or a song) and append something to it, based on taggable_type. Here's the crux.

[jnylen]

Hey @solnic,
Did you get anywhere on this?

I have some models that use polymorphism in mongoid that I want to move over.

The way I do it is:
belongs_to :media, polymorphic: true, index: true

where media can be any type of model.

What is the best way to do this? I don't want 10 different belongs_to.

[solnic]

@jnylen there's a chance it'll be supported in rom 4, if not then 4.1. It's low priority at the moment, so it needs to wait.

[solnic]

Closing this. We'll keep this in mind and eventually make this possible.