Call toFirestore() only once

.changeset/tame-donuts-relate.md

@@ -0,0 +1,5 @@
+---
+'@firebase/firestore': patch
+---
+
+Fixed a bug where CollectionReference.add() called FirestoreDataConverter.toFirestore() twice intead of once (#3742).

packages/firestore/exp/src/api/transaction.ts

@@ -29,7 +29,8 @@ import { Transaction as InternalTransaction } from '../../../src/core/transactio
 import { validateReference } from '../../../lite/src/api/write_batch';
 import { getDatastore } from '../../../lite/src/api/components';
 
-export class Transaction extends LiteTransaction
+export class Transaction
+  extends LiteTransaction
   implements firestore.Transaction {
   // This class implements the same logic as the Transaction API in the Lite SDK
   // but is subclassed in order to return its own DocumentSnapshot types.

packages/firestore/lite/src/api/field_value.ts

@@ -30,7 +30,8 @@ import { ParseContext } from '../../../src/api/user_data_reader';
 import { FieldTransform } from '../../../src/model/mutation';
 
 /** The public FieldValue class of the lite API. */
-export abstract class FieldValue extends SerializableFieldValue
+export abstract class FieldValue
+  extends SerializableFieldValue
   implements firestore.FieldValue {}
 
 /**

packages/firestore/src/api/database.ts

@@ -2379,8 +2379,18 @@ export class CollectionReference<T = DocumentData>
       ? this._converter.toFirestore(value)
       : value;
     validateArgType('CollectionReference.add', 'object', 1, convertedValue);
+
     const docRef = this.doc();
-    return docRef.set(value).then(() => docRef);
+
+    // Call set() with the converted value directly to avoid calling toFirestore() a second time.
+    // Cast to unknown in order to access the _key property.
+    return new DocumentReference(
+      ((docRef as unknown) as DocumentReference)._key,
+      this.firestore,
+      null
+    )
+      .set(convertedValue)
+      .then(() => docRef);
   }
 
   withConverter<U>(