我試圖弄清楚如何在 Firestore 的以下目錄中構建一個新的子集合（紅色圓圈）：目錄：“課程/{userID}/{courseID}”courseID 是在“userID”文檔中創建子集合時隨機生成的 ID。不幸的是，我能夠使用 .set() 運行的代碼不會創建新的子集合，而只是更新當前文檔。是否有可用于在文檔中創建新集合的函數？代碼：export const createCourse = (course) => {        (...)    //firestore.collection("courses").doc(authorId).set({   <---This commented out line updates the document, yet I'm trying to construct a new sub-collection inside this document, not just change fields inside of it.     firestore.collection("courses").doc(authorId).add({     <--- This gives me an error stating " firestore.collection(...).doc(...).add is not a function ".   I am trying to figure out what is the correct syntax for this intended action.        ...course,        authorUID: authorId,        courseCreatedAt: new Date()    }).then(() => {    (...)};