Mapped dataloader, how to expose the correct value?

[jarlehansen]

We are using data loaders extensively, and for the regular data loaders it works great with graphql-kotlin. We return a CompletableFuture<V> and it gets dispatched.

However, we need to use the mapped data loader (DataLoader.newMappedDataLoader) to map the key to the value(s). The return value for this type of data loader is Map<K, V>, which means it will be CompletableFuture<Map<K, V>>. When I add this to a graphql-kotlin query class, it fails because it says that Map is not a graphql type.

Is there any examples of doing this?
Does it not support this type of data loader, or am I doing something wrong?

[jarlehansen]

I think I might have done something wrong in the load function, but there is something I do not understand. If we have a simple return value, such as CompletableFuture<MyValue> it works. However, if I need a list of values (CompletableFuture<List<MyValue>> it gives this error:

type mismatch error, expected type LIST got class java.util.concurrent.CompletableFuture

[smyrick]

Hmm interesting on the CompletableFuture<List<MyValue>> as that should be a valid return type. I can check if that throws an error with a basic test.

On the CompletableFuture<Map<K, V>>, that is indeed an invalid return type so can you explain a little bit more how you are using the MappedDataLoader as it still should return a single Dataloader

https://javadoc.io/doc/com.graphql-java/java-dataloader/2.2.1/org/dataloader/DataLoader.html

[smyrick]

The CompletableFuture<List<String>> is valid.

@Test
fun validateSchema() {
    val queries = listOf(TopLevelObject(SimpleQuery()))
    val schema = toSchema(testSchemaConfig, queries)
    assertNotNull(schema)
    assertEquals("String!", schema.queryType.getFieldDefinition("getString").type.toString())
    assertEquals("String!", schema.queryType.getFieldDefinition("getDataFetcherResultString").type.toString())
    assertEquals("[String!]!", schema.queryType.getFieldDefinition("getListString").type.toString())
}

class SimpleQuery {
    private val string = "hello"

    fun getString(): CompletableFuture<String> = CompletableFuture.completedFuture(string)
    fun getDataFetcherResultString(): CompletableFuture<DataFetcherResult<String>> =
        CompletableFuture.completedFuture(DataFetcherResult.newResult<String>().data(string).build())
    fun getListString(): CompletableFuture<List<String>> = CompletableFuture.completedFuture(listOf(string))
}

[smyrick]

#1032

[jarlehansen]

Thanks for the examples, really useful.
What about CompletableFuture<List<DataFetcherResult<String>>>? I get an error on this:

com.expediagroup.graphql.exceptions.TypeNotSupportedException: Cannot convert graphql.execution.DataFetcherResult

Shouldnt this be allowed if you have more than one value pr key?

[smyrick]

It would be CompletableFuture<DataFetcherResult<List<String>>>

CompletableFuture and DataFetcherResult are just wrapping types, whereas List<String> is the actual type exposed in the schema. The errors field in the single DataFetcherResult is a list of errors you can append to