Incorrect query requirement for union types

[stuebingerb]

While analyzing #667 I came across UnionsSpecificationTest#a union type should require a selection for all potential types that requires selections for every possible union type:

@Test
fun `a union type should require a selection for all potential types`() {
    expectRequestError<ValidationException>("Missing selection set for type 'Scenario'") {
        testedSchema.executeBlocking(
            """{
            actors {
                name
                favourite {
                    ... on Actor { __typename }
                    ... on Director { name, age }
                }
            }
        }""".trimIndent()
        )
    }
}

However, that requirement doesn't actually seem to be backed by the spec. I wasn't able to find anything related, and the example on https://graphql.com/learn/interfaces-and-unions/#querying-with-unions also executes fine with selections for only a subset of potential types. It also doesn't seem like a similar requirement has been implemented for interfaces in KGraphQL.

[stuebingerb]

For reference: #122 (comment)

types without fragment result in an empty selection set, and according to the spec, empty selection sets are invalid.

Which kinda still makes sense, although the same would then also apply to a selection set with only skipped fields IMO which isn't handled right now either:

{
  film {
    year @skip(if: true)
  }
}

{
  "data" : {
    "film" : { }
  }
}

[stuebingerb]

See also:

• Allow empty selection sets graphql/graphql-spec#1227
• What must be the result of executing only one fragment on a list of union or interface type? graphql/graphql-spec#1080
• Hint that a field of an union or interface type will resolve only to types specified in the selection set graphql/graphql-spec#951
• GraphQL spec is contradictory: can selection sets be empty or not? graphql/graphql-spec#1024
• Clarify that selection sets cannot be empty graphql/graphql-spec#1025
• failure to require leaf field selection when using fragments graphql/graphql-spec#610

In particular, graphql/graphql-spec#1080 explicitly states that the observed behavior is correct (i.e. there is no requirement to implement a fragment for every possible type), and graphql/graphql-spec#1227 plans to allow empty selection sets in general. According to graphql/graphql-spec#610 empty selection sets weren't introduced to prevent empty objects in general but to guard against unintentionally empty objects due to user errors.

[stuebingerb]

tl;dr: allowing only partial selections seems confusing but is in fact how it is supposed to work. Therefore, the requirement in KGraphQL is indeed a bug.

Instead, the query in the test should return the following (note the empty object for "Morgan Freeman"):

{
  "data" : {
    "actors" : [ {
      "name" : "Brad Pitt",
      "favourite" : {
        "__typename" : "Actor"
      }
    }, {
      "name" : "Morgan Freeman",
      "favourite" : { }
    }, {
      "name" : "Kevin Spacey",
      "favourite" : {
        "__typename" : "Actor"
      }
    }, {
      "name" : "Tom Hardy",
      "favourite" : {
        "name" : "Christopher Nolan",
        "age" : 43
      }
    }, {
      "name" : "Christian Bale",
      "favourite" : {
        "__typename" : "Actor"
      }
    } ]
  }
}