Query and where

Filter queries with where.

Filtering with where

The where method filters the documents in a collection by a field. For example where('sender', isEqualTo: 'Ali') returns only documents whose sender is Ali. where returns a Query; you call snapshots or get on it.

db.collection('messages')
  .where('sender', isEqualTo: 'Ali')
  .snapshots();

Chained queries

You can chain multiple where calls to build more specific filters. You can add ordering with orderBy and limit the document count with limit. The query object is immutable, so each call returns a new query.

db.collection('messages')
  .where('sender', isEqualTo: 'Ali')
  .where('read', isEqualTo: false)
  .snapshots();

Exercise Listen to a filtered query

Get db. Apply where('sender', isEqualTo: 'Ali') on the 'messages' collection and listen to the snapshots stream. Print the document count on each snapshot. Wait 10 milliseconds.

Show example solution

class QueryDocumentSnapshot {
  final Map<String, dynamic> data;
  QueryDocumentSnapshot(this.data);
}

class QuerySnapshot {
  final List<QueryDocumentSnapshot> docs;
  QuerySnapshot(this.docs);
}

class Query {
  final List<QueryDocumentSnapshot> docs;
  Query(this.docs);
  Stream<QuerySnapshot> snapshots() async* {
    yield QuerySnapshot(docs);
  }
}

class CollectionReference {
  final String name;
  CollectionReference(this.name);
  Query where(String field, {required String isEqualTo}) {
    return Query([
      QueryDocumentSnapshot({'sender': isEqualTo, 'text': 'Merhaba'}),
    ]);
  }
}

class FirebaseFirestore {
  static final FirebaseFirestore instance = FirebaseFirestore._();
  FirebaseFirestore._();
  CollectionReference collection(String name) => CollectionReference(name);
}

void main() async {
  final db = FirebaseFirestore.instance;
  db.collection('messages')
    .where('sender', isEqualTo: 'Ali')
    .snapshots()
    .listen((snap) {
      print(snap.docs.length);
    });
  await Future.delayed(Duration(milliseconds: 10));
}

Test yourself

What does where('sender', isEqualTo: 'Ali') do?
- Deletes all documents
- Selects only documents whose sender field is Ali
- Orders documents by date
- Creates a new collection
Show answer

Selects only documents whose sender field is Ali — where with isEqualTo filters documents equal to the given value; it does not order or delete.

Add a where filter

db.collection('messages')
  .____('sender', isEqualTo: 'Ali')
  .snapshots();

Show answer

db.collection('messages')
  .where('sender', isEqualTo: 'Ali')
  .snapshots();

Query parts

where

Filters by a field
isEqualTo

Equality condition
Query

Query object
snapshots

Produces a stream

Next lesson: Summary - Firestore

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