public Optional<String> getType(Set<String> streamIds, String field) {
        final Map<String, Set<String>> allFieldTypes = this.get(streamIds);
        final Set<String> fieldTypes = allFieldTypes.get(field);
        return typeFromFieldType(fieldTypes);
    }
@Test
    void returnsEmptyOptionalIfMultipleTypesExistForField() {
        final Pair<IndexFieldTypesService, StreamService> services = mockServices(
                IndexFieldTypesDTO.create("indexSet1", "stream1", ImmutableSet.of(
                        FieldTypeDTO.create("somefield", "long")
                )),
                IndexFieldTypesDTO.create("indexSet2", "stream1", ImmutableSet.of(
                        FieldTypeDTO.create("somefield", "float")
                ))
        );

        final FieldTypesLookup lookup = new FieldTypesLookup(services.getLeft(), services.getRight());
        final Optional<String> result = lookup.getType(Collections.singleton("stream1"), "somefield");
        assertThat(result).isEmpty();
    }