你可以使用QuerySnapshot的orderBy方法和descending true。例如，你有一个集合documents，你希望它按uids排序

CollectionReference documentsRef = FirebaseFirestore.instance.collection('documents');

QuerySnapshot snapshot = await documentsRef
    .orderBy('uids', descending: true)
    .get();

List<QueryDocumentSnapshot> documents = snapshot.docs;

使用Cloud firestore的orderBy方法根据数组字段的长度对文档进行排序，如下所示

const db = firebase.firestore();
const collectionRef = db.collection('udid_collection');

// Sort documents in descending order based on the length of the "uids" array field
collectionRef.orderBy('uids', 'desc').get()
  .then(querySnapshot => {
    querySnapshot.forEach(doc => {
      console.log(doc.data().uids);
    });
  })
  .catch(error => {
    console.log(error);
  });