Gateway will merge incorrect data if subgraph changes entity order

[smyrick]

Issue Description

An user is building a subgraph with the Ruby library that supports federation. As part of this library, they don't just support a resolve_reference function to resolve a single entity reference but they also have support to resolve multiple with resolve_references (plural): Gusto/apollo-federation-ruby#206

With this new feature, the subgraph now gets the entire entity array and is asked to resolve the specific fields.

[
  {"__typename"=>"Product", "upc"=>"1"},
  {"__typename"=>"Product", "upc"=>"2"},
  {"__typename"=>"Product", "upc"=>"3"}
]

Normally you would only be called once for each entity and the array order could never change but with this method you could now change the order of the elements. You can still keep the ids the same but you can now return a response to the gateway with the fields filled out but the order changed from what the gateway sent

[
  {"__typename"=>"Product", "upc"=>"3", "inStock": true}, 
  {"__typename"=>"Product", "upc"=>"2",  "inStock": true},
  {"__typename"=>"Product", "upc"=>"1", "inStock": false},
]

In theory the Gateway should be able to do a id match but instead it assume the order is the same so the data returned to clients shows the Product:1 in stock is true and Product:3 in stock is false which is not correct

{
  "data": {
    "topProducts": [
      {
        "upc": "1",
        "inStock": true
      },
      {
        "upc": "2",
        "inStock": true
      },
      {
        "upc": "3",
        "inStock": false
      }
    ]
  }
}

Link to Reproduction

https://github.com/Gusto/apollo-federation-ruby/compare/main...smyrick:apollo-federation-ruby:shane-gateway-bug?expand=1

Reproduction Steps

• Start up the example server and gateway here
  • yarn start-services
  • yarn start-gateway
• Run the following query

  query ExampleQuery {
    topProducts {
      upc
      inStock
    }
  }

See how the data is mismatched

The data is defined like so

INVENTORY = [
  { upc: '1', in_stock: true },
  { upc: '2', in_stock: true },
  { upc: '3', in_stock: false },
]

but the client sees this

{
  "data": {
    "topProducts": [
      {
        "upc": "1",
        "inStock": false
      },
      {
        "upc": "2",
        "inStock": true
      },
      {
        "upc": "3",
        "inStock": true
      }
    ]
  }
}

[smyrick]

After discussing with the Apollo team, this is actually expected behaviour for Federation, and there fore we are going to close a won't fix.

I will however open up an issue on the Ruby library to maybe see if there is something they can implement on their end since this is a non-standard way of exposing the entities array

https://www.apollographql.com/docs/federation/subgraph-spec/#understanding-query_entities