resolveField

Resolves a field from the cache. Called when reading from the cache, usually before a network request.

• takes a GraphQL field and operation variables as input and generates data for this field

• this data can be a CacheKey for objects but it can also be any other data if needed. In that respect, it's closer to a resolver as might be found in apollo-server

• is used before a network request

• is used when reading the cache

It can be used to map field arguments to CacheKey:

{
  user(id: "1"}) {
    id
    firstName
    lastName
  }
}

override fun resolveField(field: CompiledField, variables: Executable.Variables, parent: Map<String, Any?>, parentId: String): Any? {
  val id = field.resolveArgument("id", variables)?.toString()
  if (id != null) {
    return CacheKey(id)
  }

  return super.resolveField(field, variables, parent, parentId)
}

The simple example above isn't very representative as most of the times @fieldPolicy can express simple argument mappings in a more concise way but still demonstrates how resolveField works.

resolveField can also be generalized to return any value:

override fun resolveField(field: CompiledField, variables: Executable.Variables, parent: Map<String, Any?>, parentId: String): Any? {
  if (field.name == "name") {
    // Every "name" field will return "JohnDoe" now!
    return "JohnDoe"
  }

  return super.resolveField(field, variables, parent, parentId)
}

See also @fieldPolicy See also CacheKeyGenerator

Parameters