title: Flutter firestore string convert to document reference for where clause
body: <p>My chat collection has a field event_id and its type is reference now I want to use my events document id for query how can I convert string to DocumentReference type so I can pass into where clause and query through the chat collection</p>
  <p>chat.where('event_id', isEqualTo:'events/J3HTFRpL0H').snapshots(),</p>