Firestore unique index or unique constraint?

Question

Is it possible in Firestore to define an index with a unique constraint? If not, how is it possible to enforce uniqueness on a document field (without using document ID)?

Answer 1

[Its not a perfect solution but working]

I have done this unique key using key... I want my table to be having unique date value. so i made it key of my document. Any way i am able to get all documents

db.collection('sensors').doc(sensorId).collection("data").doc(date).set(dataObj).then(() => {
    response.send(dataObj);
});