How to test federation _entities endpoint with the generated (test) clients?

[roookeee]

We are currently in a bind because we cannot test the _entities federation endpoint when implementing FederatedTypeResolver<T> via a GraphQL query that is used for generating a test client via the gradle plugin.

The _Any type is typealiasd to String but the represesentations [_Any!]! is expected to be a List<Map<String,Any>> in the underlying framework code. Defining custom ScalarConverter<T> for _Any does not work as the representation array inner values are always treated as String when sending it via the generated clients even when we do custom conversion in a ScalarConverter.

As far as I know scalar types in GraphQL are always implemented with String as its underlying type which does not hold true for the special _Any type of federation in conjunction with the _entities endpoint.

Doing an API test for the crucial _entities endpoints at the GraphQL-query-layer seems like the way to go when doing application testing but we are currently forced to test the FederatedTypeResolver<T> directly which is a bit of a bummer.

Example:

query FederationTest($reps: [_Any!]!) {
    _entities(representations: $reps) {
        ... on SomeData {
           key,
           someField
        },
        __typename
    }
} 	

will generate the following query in the generated clients even when defining a custom scalar converter which interprets the _Any Strings as a map via Jackson and then converts it into a proper JSON object / map via the same ObjectMapper:

    {
        "variables": {
            "reps": [
                "{\"__typename\":\"SomeData\",\"key\":\"1\"}",
                "{\"__typename\":\"SomeData\",\"key\":\"2\"}",
                "{\"__typename\":\"SomeData\",\"key\":\"3\"}",
            ]
        },
        "query": "...", //omitted
   }

compared to the correct:

    {
        "variables": {
            "reps": [
                {
                     "__typename":"SomeData",
                     "key":"1"
                },
                {
                     "__typename":"SomeData",
                     "key":"2"
                },                
                {
                     "__typename":"SomeData",
                     "key":"3"
                },
            ]
        },
        "query": "...", //omitted
   }

We would appreciate any help / a way to test these federation endpoints via the test clients as certain contracts (e.g. not throwing exceptions when a single key cannot be resolved in a batch federation request) are testworthy and starting on the GraphQL layer is more thorough than actually invoking the FederatedTypeResolver<T> directly (which also needs a DataFetchingEnvironment).

[roookeee]

Just ran into another nasty misuse of graphql-kotlin that could have been detected if this was possible: Someone defined a custom FederatedSchemaGeneratorHooks and just provided a listOf() instead of autowiring the FederatedTypeResolver<T> into it which caused them not to be registered - we would have noticed that issue if our test had used the _entities endpoint instead of directly calling our FederatedTypeResolver<T>

[dariuszkuc]

See #1488. I've updated example apps and docs with an example.

[dariuszkuc]

Resolved by #1488