ObjectCache: how to handle child objects without their own id?

[ludo]

We're setting up ObjectCache in our GraphQL API. One thing I'm running into is that we have a few GraphQL types that are virtual (not backed by an identifiable object) and I'm not sure how to deal with them. If I do nothing GraphQL Ruby will be unhappy because id_from_object will cause an exception.

It seems like Caching lists and connections and Object Dependencies are relevant to this problem but I can't wrap my head around how to apply them. See code below, I think I'm looking for a way to call the parent object in FormFieldType.

Here's an excerpt of the code I'm working with:

module Types
  class FormType < Types::BaseObject
    field :fields, [Types::FormFieldType], null: false
  end
  
  class FormFieldType < Types::BaseObject
    field :name, String, null: false
  end
end

FormType resolves to a Form ActiveRecord model. This model has a fields attribute which is an array of hashes (JSON in the database). For example: #<Form id: 1, fields: [{"name"=>"FOO"}, {"name"=>"BAR"}]>.

At this point when we query a form with its fields it will raise an error because fields aren't 'real' objects with ids:

query {
  forms {
    nodes {
      fields {
        name
      }
    }
  }
}

Failure/Error: Base64.strict_encode64(object.to_global_id.to_s)

NoMethodError:
  undefined method `to_global_id' for #<...>

I kind of just want to tell FormFieldType to be cached as part of its parent if that makes sense?

[rmosolgo]

Hey, thanks for the great question.

Yes, I think cache_dependencies_for is the way to go, you could implement it like this:

class Types::FormFieldType < Types::BaseObject
  def self.cache_dependencies_for(form_field, context)
    form_field.parent 
  end  
  
  # ... 
end 

Can you return the form field's form object easily? The most fail-safe would be to use an application-defined method on the form field (or add one if necessary, add "form" => form to the hash, etc). That way, you could be sure that the form field would have access to the right parent, no matter what.

In a pinch, you could use GraphQL-Ruby's "scoped context" field to accomplish this, but I don't recommend it because scoped context breaks if the query doesn't meet the assumptions of the Ruby code. (That is, if the incoming query was a different structure than assumed, weird things can happen since the query tree was different.) In any case, here it is: https://graphql-ruby.org/queries/executing_queries.html#scoped-context

Does that help?

[ludo]

Can you return the form field's form object easily? The most fail-safe would be to use an application-defined method on the form field (or add one if necessary, add "form" => form to the hash, etc). That way, you could be sure that the form field would have access to the right parent, no matter what.

Ah, thanks, that's a good idea. I was initially trying to access the parent through context (I don't think that's available?). Your suggestion seems fine though. This works and is simple enough:

module Types
  class FormType < Types::BaseObject
    field :fields, [Types::FormFieldType], null: false
    def fields
      object.fields.map { |field| field.merge({ parent: object }) }
    end
  end
  
  class FormFieldType < Types::BaseObject
    def self.cache_dependencies_for(form_field)
      form_field[:parent]
    end

    field :name, String, null: false
  end
end

Thanks for your help!