Firebase: fetch existing data once, then use child_added to get new data

Question

I'd like to just fetch a list once and then have it in memory and use child_added to process items being created.

It's inconvenient for me to only use child_added because doing so would cause me to re-render my entire list for every child_added event. I'd rather render the whole list once at page load, then for each item that's added.

My code:

ref.once('value', (snapshot) => { let messages = [] snapshot.forEach(function(childSnapshot) { let messageKey = childSnapshot.key let message = childSnapshot.val() messages.push(message) }) // Render messages list once ref.on('child_added', (message) => { const messageVal = message.val() // Append message and re-render list } })

The problem is that I still get child_added events for each existing node.

Is the most advisable thing to do just to track which items I've already processed and ignore them in child_added ? Or is there a way to only get new updates?

Thanks

Doug Stevenson · Accepted Answer · 2018-02-07 18:50:28Z

If you introduce a way to determine exactly which items are "new" at any given point in time, you can use that in your 'child_added' query to pick up where a 'value' query left off. For example, if your data items have a timestamp associated with each item, and you know the latest timestamp from all the items you received from a 'value' query, modify the 'child_added' query to filter for only items >= that timestamp.

If your data doesn't help you create a natural time-based ordering of your items like this, or you've already exhausted your one allowed filter for the RTDB query, I don't think there's much else you can do (except maybe move to Firestore, which gives you multiple filters).

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