Problem serializing Postgres' array columns due to delegation.

[tomasmiguez]

Hi!
We are currently working with Sequel as our ORM, Postgres as our DBMS and Oj for dumping objects to JSON. This works fine on most use cases but we recently started using array type columns and started experiencing some issues.

For example, if we have this table

                Table "public.tests"
 Column |   Type    | Collation | Nullable | Default
--------+-----------+-----------+----------+---------
 foo    | integer[] |           |          |

and the model (the pg_array extension is already loaded)

require 'sequel'
  
class Test < Sequel::Model
end

when accesing the column we get:

irb:004> Test.first.foo
=> [1, 2, 3, 4, 5]

but that's not an array, it's a PGArray that delegates to Array. This causes a problem when dumping with Oj, as that gem serializes the instance variables of the object that is being dumped, and we get

irb:006> puts Oj.dump(Test.first.foo)
{"^o":"Sequel::Postgres::PGArray","delegate_dc_obj":[1,2,3,4,5],"array_type":":integer"}
=> nil

which is not the expected result. We checked the source code of the extension from where we got the idea to try to solve this issue by registering a conversion proc with add_conversion_proc, but without success. We also tried different option on the dumping of Oj, mainly use_to_hash and use_to_json but this didn't help.

The current solution we are using is to explicitly get the delegate object of the column on the accesor

  class Test < Sequel::Model
    def foo
      super.__getobj__
    end
  end

which works, but we would rather not have to override that for every array column of every model.

Is there any mechanism in Sequel that might help us solve this issue? Maybe a way to wrap the accesors methods of every column of every model for a specific type or something like that?

Thanks!

[jeremyevans]

I don't think something is built into Sequel that will do what you want. Having the access method return the underlying array instead of the wrapper could definitely break things in certain cases (though maybe not cases that would affect you).

I've never used Oj, so I don't know how much control it gives over the dumping process. It would be best to fix the issue in Oj, since the problem is Oj not handling delegate objects as you want (this isn't a Sequel-specific issue, it likely affects any use of the delegate classes using the stdlib delegate library). Does setting the create_id: nil option when calling Oj.dump fix things?

[tomasmiguez]

Thanks Jeremy! We will try that right away or check if we can fix it from Oj's side.

[rgalanakis]

I know this doesn't directly answer your question, but we use PG array columns and Oj in webhookdb. It took some fiddling to get the JSON dumping right (all the knobs are sort of confusing), here is the code we ended up with, in case it's useful: https://github.com/webhookdb/webhookdb/blob/main/lib/webhookdb/json.rb

[tomasmiguez]

Thanks for the info! We would like to keep custom logic for this to a minimum and use Oj's logic as much as we can for performance reasons, but we will keep this in mind as an option.