### Question:
AbstractMongoCollection implements MongoCollection<P> { Object deriveDocumentId(Document selector) { Object value = selector.get(getIdField()); if (value != null) { if (!Utils.containsQueryExpression(value)) { return value; } else { return deriveIdFromExpression(value); } } return new ObjectId(); } protected  AbstractMongoCollection(MongoDatabase database, String collectionName, CollectionOptions options,
                                      CursorRegistry cursorRegistry); @Override void addDocument(Document document); @Override MongoDatabase getDatabase(); @Override String getCollectionName(); @Override String toString(); @Override void addIndex(Index<P> index); @Override void drop(); @Override void dropIndex(String indexName); @Override Document findAndModify(Document query); @Override QueryResult handleQuery(QueryParameters queryParameters); @Override List<Document> insertDocuments(List<Document> documents, boolean isOrdered); @Override Document handleDistinct(Document query); @Override int deleteDocuments(Document selector, int limit, Oplog oplog); @Override Document updateDocuments(Document selector, Document updateQuery, ArrayFilters arrayFilters,
                                    boolean isMulti, boolean isUpsert, Oplog oplog); @Override List<Index<P>> getIndexes(); @Override int count(Document query, int skip, int limit); @Override Document getStats(); @Override void removeDocument(Document document); @Override Document validate(); @Override void renameTo(MongoDatabase newDatabase, String newCollectionName);  }### Answer:
@Test void testDeriveDocumentId() throws Exception { assertThat(collection.deriveDocumentId(json(""))).isInstanceOf(ObjectId.class); assertThat(collection.deriveDocumentId(json("a: 1"))).isInstanceOf(ObjectId.class); assertThat(collection.deriveDocumentId(json("_id: 1"))).isEqualTo(1); assertThat(collection.deriveDocumentId(json("_id: {$in: [1]}"))).isEqualTo(1); assertThat(collection.deriveDocumentId(json("_id: {$in: []}"))).isInstanceOf(ObjectId.class); }