`nested` tries to assign to non-existent property when parsing nil body

[jestraw]

`nested` tries to assign to non-existent property when parsing nil body

[jestraw]

For what it's worth (and I've been using representable since 2014 or so), the documentation for nested implies that it's a way of accommodating some structure of the document you are parsing which doesn't match your object structure. In my (apparently inaccurate) mental model of what is going on, nested is just for handling parsing or rendering of the document. It was extremely surprising to have it suddenly acting like property and trying to assign a null value to a non-existent attribute on the represented object!

[jestraw]

I've created a file to test this behavior in different combinations: https://gist.github.com/jestraw/7a88cf42984db2e5e3d560ea58970a11

Running this file against 3.1.1 gives the following output:

JsonSongRepresenter
{                                        
  "title": "json-with-details",          
  "details": {                           
    "track": 1                           
  }                                      
}                                        
#<struct Song title="json-with-details", track=1, length=nil>
                                         
{                                        
  "title": "json-empty-details",         
  "details": { }                         
}                                        
#<struct Song title="json-empty-details", track=nil, length=nil>

{
  "title": "json-null-details",
  "details": null
}
undefined method `details=' for #<struct Song title="json-null-details", track=nil, length=nil>

HashSongRepresenter
{"title"=>"hash-with-details", "details"=>{"track"=>1}}
#<struct Song title="hash-with-details", track=1, length=nil>

{"title"=>"hash-empty-details", "details"=>{}}
#<struct Song title="hash-empty-details", track=nil, length=nil>

{"title"=>"hash-null-details", "details"=>nil}
undefined method `details=' for #<struct Song title="hash-null-details", track=nil, length=nil>

JsonHashSongRepresenter
{"title"=>"hash-with-details", "details"=>{"track"=>1}}
#<struct Song title="hash-with-details", track=1, length=nil>

{"title"=>"hash-empty-details", "details"=>{}}
#<struct Song title="hash-empty-details", track=nil, length=nil>

{"title"=>"hash-null-details", "details"=>nil}
undefined method `details=' for #<struct Song title="hash-null-details", track=nil, length=nil>

[apotonick]

Hi @jestraw! The problem here is that Song doesn't have a details= method!

[jestraw]

Yes, I know. But why is nested trying to assign to the represented object?

[apotonick]

That's the design of Representable, in Reform (also using Representable), we have "populators" to solve this problem. When parsing, it simply writes the read values on represented. You'd probably need the :class option here. Hit me up on https://trailblazer.zulipchat.com if you need assistance! 🍻

It won't help you now but I will soon start working on Representable 4 which is, exactly like Reform 3, completely based on Trailblazer::Activity meaning you can customize it much easier.

[apotonick]

@jestraw nested will read the nested fragment, may or may not create a nested "target" object for that and than assign that to represented - in your mental model, what would Rpr do with the parsed fragment instead?

[jestraw]

Fair enough! I think the actual document I was trying to parse was weird -- it was essentially relating a kind of left outer join where the joined table was empty by presenting the key for the join with a nil value. Since the document was already parsed to a hash, I could have done .compact and gotten rid of the problem, but that wouldn't have looked as nice as just doing for_collection. I ended up with some if directives on the nested to skip parsing if the value was nil, which does work.