Firestore: CollectionReference.withConverter(converter).add(data) invokes converter.toFirestore twice

[hecateball]

[REQUIRED] Describe your environment

• Operating System version: macOS Catalina 10.15.6
• Browser version: N/A
• Firebase SDK version: 7.19.1
• Firebase Product: firestore

[REQUIRED] Describe the problem

firebase.firestore().collection('...').withConverter(converter).add(data) invokes converter.toFirestore twice while firebase.firestore().collection('...').withConverter(converter).doc().set(data) invokes converter.toFirestore only once.
Is this intended behavior?

Although this is not a desirable style of processing, if the toFirestore() process has a side effect on the input value, the second invocation fails (See example below).

Steps to reproduce:

try to run following code please.

Relevant Code:

const firebase = require('firebase')

// Please use your firebase config here
firebase.initializeApp(firebaseConfig)

const document = {
  someArray: [
    { text: '' },
  ],
}

const converter = {
  toFirestore: (modelObject) => {
    console.log('toFirestore', modelObject)
    modelObject.someArray = 'text'
    return { text: modelObject.someArray[0].text }
  },
  fromFirestore: () => {
    throw new ReferenceError()
  },
}

// log 'toFirestore' twice, and then fail second conversion.
firebase.firestore().collection('posts')
  .withConverter(converter)
  .add(document)

// log 'toFirestore' only once and complete as expected.
firebase.firestore().collection('posts')
  .withConverter(converter)
  .doc()
  .set(document)

[thebrianchen]

@hecateball Thanks for filing the bug with an easy repro! We should have a fix for this out in the coming weeks.