Firestore - How to get document id after adding a document to a collection

Question

Is there a way to acquire the document id that was generated after adding a document to a collection?

If I add a document to a collection that represents a "post" in a social media app, I want to get that document id and use it as a field in another document in a different collection.

If I can't get the document Id that was generated after adding a document, should I just compute a random string and supply the id when creating the document instead? That way I can use that same string as the field in my other document?

Quick structure example:

POST (collection)     Document Id - randomly generated by firebase or by me USER (collection)     Document Id - randomly generated by firebase        userPost: String (this will be the document id                           in the post collection that I'm trying to get) 

Answer 1

Yes it is possible. When you call the .add method on a collection, a DocumentReference object is returned. DocumentReference has the id field, so you can get the id after the document was created.

// Add a new document with a generated id. db.collection("cities").add({     name: "Tokyo",     country: "Japan" }) .then(function(docRef) {     console.log("Document written with ID: ", docRef.id); }) .catch(function(error) {     console.error("Error adding document: ", error); }); 

This example is in JavaScript. Visit the documentation for other languages.

Answer 2

If using promises, I'd recommend using fat arrow function as it opens up the possibility for using this.foo even in the .then function

    db.collection("cities").add({         name: "Tokyo",         country: "Japan"     })     .then(docRef => {         console.log("Document written with ID: ", docRef.id);         console.log("You can now also access this. as expected: ", this.foo)     })     .catch(error => console.error("Error adding document: ", error)) 

Using function(docRef) means you cannot access this.foo, and error will be thrown

    .then(function(docRef) {         console.log("Document written with ID: ", docRef.id);         console.log("You can now NOT access this. as expected: ", this.foo)     }) 

While fat arrow functions will allow you to access this.foo as expected

    .then(docRef => {         console.log("Document written with ID: ", docRef.id);         console.log("You can now also access this. as expected: ", this.foo)     }) 

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Edit/addition 2020:

A more popular way these days may be to use async/await instead. Notice that you have to add async in front of the function declaration:

    async function addCity(newCity) {       const newCityAdded = await db.collection("cities").add(newCity)       console.log("the new city:", newCityAdded)       console.log("it's id:", newCityAdded.id)     } 

And if you only want the id it can be grabbed using descructuring. Destructuring allows you to grab any key/value-pair in the response:

    async function addCity(newCity) {       const { id } = await db.collection("cities").add(newCity)       console.log("the new city's id:", id)     } 

It's also possible to use destructuring to grab the value and rename to whatever you want:

    async function addCity(newCity) {       const { id: newCityId } = await db.collection("cities").add(newCity)       console.log("the new city's id:", newCityId)     }