How to add timestamp to every collection insert,update in Cloud Functions for firestore database

Question

I have a firestore collection called Posts I make an insert on the client side and it works.

I want to add the createdAt and updatedAt fields to every insert in my p

Answer 1

This solution supports first-level subcollections and is based off @Jonathan's answer above:

**
 * writes fields common to root-level collection records that are generated by the
 * admin SDK (backend):
 * - createdAt (timestamp)
 * - updatedAt (timestamp)
 */
exports.createCommonFields = functions.firestore
.document('{colId}/{docId}')
.onWrite(async (change, context) => {
    // the collections you want to trigger
    const setCols = ['posts', 'reviews', 'comments', ];

    // run the field creator if the document being touched belongs to a registered collection
    if (setCols.includes(context.params.colId)) {
        console.log(`collection ${context.params.colId} is not registered for this trigger`);
        return null;
    } else {
        console.log(`running createCommonFields() for collection: ${context.params.colId}`);
    }

    // cause the creation of timestamp fields only
    _createCommonFields(change);
});

/**
 * createCommonFields' equivalent for sub-collection records
 */
exports.createCommonFieldsSubColl = functions.firestore
.document('{colId}/{colDocId}/{subColId}/{subColDocId}')
.onWrite(async (change, context) => {
    console.log(`collection: ${context.params.colId}, subcollection: ${context.params.subColId}`);

    // the subcollections of the collections you want to trigger
    // triggers for documents like 'posts/postId/versions/versionId, etc
    const setCols = {
        'posts': ['versions', 'tags', 'links', ], 
        'reviews': ['authors', 'versions'],
        'comments': ['upvotes', 'flags'],
    };

    // parse the collection and subcollection names of this document
    const colId = context.params.colId;
    const subColId = context.params.subColId;
    // check that the document being triggered belongs to a registered subcollection
    // e.g posts/versions; skip the field creation if it's not included
    if (setCols[colId] && setCols[colId].includes(subColId)) {
        console.log(`running createCommonFieldsSubColl() for this subcollection`);
    } else {
        console.log(`collection ${context.params.colId}/${context.params.subColId} is not registered for this trigger`);
        return null;
    }

    // cause the creation of timestamp fields
    _createCommonFields(change);
});

/**
 * performs actual creation of fields that are common to the
 * registered collection being written
 * @param {QueryDocumentSnapshot} change a snapshot for the collection being written
 */
async function _createCommonFields(change) {
    // simplify event types
    const createDoc = change.after.exists && !change.before.exists;
    const updateDoc = change.before.exists && change.after.exists;
    const deleteDoc = change.before.exists && !change.after.exists;

    if (deleteDoc) {
        return null;
    }

    // simplify input data
    const after = change.after.exists ? change.after.data() : null;
    const before = change.before.exists ? change.before.data() : null;

    // prevent update loops from triggers
    const canUpdate = () => {
        // if update trigger
        if (before.updatedAt && after.updatedAt) {
            if (after.updatedAt._seconds !== before.updatedAt._seconds) {
                return false;
            }
        }
        // if create trigger
        if (!before.createdAt && after.createdAt) {
            return false;
        }
        return true;
    }

    const currentTime = admin.firestore.FieldValue.serverTimestamp();
    // add createdAt
    if (createDoc) {
        return change.after.ref.set({
            createdAt: currentTime,
            updatedAt: currentTime,
        }, { merge: true })
        .catch((e) => {
            console.log(e);
            return false;
        });
    }
    // add updatedAt
    if (updateDoc && canUpdate()) {
        return change.after.ref.set({
            updatedAt: currentTime,
        }, { merge: true })
        .catch((e) => {
            console.log(e);
            return false;
        });
    }
    return null;
}