How can I increment a value from firebase?

Question

I'm trying to elaborate a function where when the icon is pressed the firebase number would increment by 1, when the icon was pressed again it would increment by -1 and it would show de total of likes returning as a "Text().toString()". I'm trying to use "FieldValue.increment" but unfortunately the number has not being increased on firebase. What am I doing wrong? thanks in advance for your help!

class LanchonetesContact extends StatefulWidget { final DocumentSnapshot lanchonetes; LanchonetesContact(this.lanchonetes); @override _LanchonetesContactState createState() => _LanchonetesContactState(); } class _LanchonetesContactState extends State<LanchonetesContact> { bool liked = false; _pressed() { setState(() { liked = !liked; }); } @override Widget build(BuildContext context) { return Scaffold( appBar: AppBar( leading: IconButton( icon: Icon(Icons.arrow_back), onPressed: () { Navigator.pop(context); }), title: Text("Lanchonetes"), centerTitle: true, backgroundColor: Colors.purple[900], actions: <Widget>[ IconButton( icon: Icon( liked ? Icons.favorite : Icons.favorite_border, color: Colors.white,) , onPressed: () { _pressed(); if(liked){ Firestore.instance.collection('lanchonetes') .document("") .updateData({"likes": FieldValue.increment(1)}); }else { Firestore.instance.collection('lanchonetes') .document("") .updateData({"likes": FieldValue.increment(-1)}); } } )], ), Padding( padding: EdgeInsets.only(top: 3.0), child: Card( elevation: 1.0, child: GestureDetector( child: Container( height: 100.0, width: 500.0, color: Colors.white, child: Padding( padding: EdgeInsets.symmetric( vertical: 30.0, horizontal: 15.0), child: Row( mainAxisAlignment: MainAxisAlignment.center, children: <Widget>[ Icon(Icons.favorite, color: Colors.black,), Text(widget.lanchonetes.data["likes"].toString()), ], ), ), ), onTap: () { }, )), ),

Loolooii · Accepted Answer · 2019-06-18 13:31:08Z

I don't see what document you are trying to update. You need the document reference.

Update: FieldValue.increment works just fine and is the shortest way to do it.

// Get document reference final DocumentReference docRef = Firestore.instance.collection("lanchonetes").document(REPLACE_WITH_ID); docRef.updateData({"likes": FieldValue.increment(1)});

Another way is to create a transaction for it:

final TransactionHandler transactionHandler = (Transaction tran) { tran.get(docRef).then((DocumentSnapshot snap) { if (snap.exists) { tran.update(docRef, <String, dynamic>{'likes': snap.data['likes'] + 1}); // update state by calling _pressed() or anything else you like } }); }; Firestore.instance.runTransaction(transactionHandler);

This would be your like() method, for dislike() you obviously replace snap.data['likes'] + 1 with snap.data['likes'] - 1

Good catch about the empty document ID. But note that a transaction is not needed. The FieldValue.increment() operator is already protected against overwrites from different users. It pretty much runs a mini-transaction on the database-level if you will.
@FrankvanPuffelen Thanks for the info, I will update my answer accordingly.
Thanks Guys it really worked! In this case I didn't specify the documentID cuz I'd like it to happen to all the documents in the collection.
@Ricardo: Firestore doesn't have an equivalent of SQLs UPDATE lanchonetes SET ... WHERE .... Do update a document, you need to know the full path to that specific document. So to update each document, you will need to know the full path to each document. This typically means you need to read the documents, loop over the query snapshot, and then update them one by one, or on batches.
@FrankvanPuffelen I though it was possible, thanks for the info, you're a pro!!

Ricardo Oscar Kanitz · Accepted Answer · 2020-01-05 04:15:42Z

After sometime I returned to this same situation and after some research I was able to find an answer. I just needed to specify my document as "widget.lanchonetes.documentId", so the firebase would read the data according to the document I choose and it was not necessary to specify it.

Firestore.instance.collection('lanchonetes') .document(widget.lanchonetes.documentId) .updateData({"likes": FieldValue.increment(1)});

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