Adding documents (add vs set) in Firebase - Visual Side-by-Side Comparison

Process Flow - Adding documents (add vs set)

Start

↓

Choose collection

↓

Auto-generate ID

↓

Save data

↓

Document saved

↓

End

You start by choosing a collection. Using add creates a new document with an auto ID. Using set saves data to a document with a specified ID.

Execution Sample

Firebase

const docRef = await collectionRef.add({name: 'Alice'});
// vs
await collectionRef.doc('user123').set({name: 'Bob'});

Shows adding a document with auto ID vs setting a document with a specific ID.

Process Table

Step	Method	Document ID	Action	Result
1	add	auto-generated	Create new doc with auto ID	Document created with new ID
2	set	'user123'	Create or overwrite doc with ID 'user123'	Document created or replaced
3	add	auto-generated	Add another doc with auto ID	New document created with different ID
4	set	'user123'	Overwrite doc with ID 'user123'	Document data replaced
5	end	-	-	No more actions

💡 No more document operations, process ends.

Status Tracker

Variable	Start	After 1	After 2	After 3	After 4	Final
docRef.id	undefined	auto-generated ID 1	auto-generated ID 1	auto-generated ID 1	auto-generated ID 1	auto-generated ID 1
collectionRef.docs	empty	1 doc (auto ID 1)	2 docs (auto ID 1, 'user123')	3 docs (auto ID 1, 'user123', auto ID 2)	3 docs (auto ID 1, 'user123' updated, auto ID 2)	3 docs (final state)

Key Moments - 2 Insights

Why does add create a new document each time but set can overwrite?

Because add auto-generates a unique ID for each document (see step 1 and 3), while set uses the specified ID and replaces data if the document exists (see step 2 and 4).

Can set create a document if it doesn't exist?

Yes, set creates the document if it does not exist, as shown in step 2 where 'user123' is created.

Visual Quiz - 3 Questions

Look at the execution table, what is the document ID created by the first add method?

A auto-generated ID 1

B 'user123'

C auto-generated ID 2

D undefined

💡 Hint

Check row 1 under Document ID in the execution_table.

At which step does the document with ID 'user123' get overwritten?

A Step 1

B Step 2

C Step 4

D Step 3

💡 Hint

Look at the Action and Result columns for 'user123' in the execution_table.

If you want to add multiple documents without specifying IDs, which method should you use?

A set

B add

C doc

D update

💡 Hint

Refer to the concept_flow where add auto-generates IDs for new documents.

Concept Snapshot

add(): Adds a new document with an auto-generated ID.
set(): Creates or overwrites a document with a specified ID.
Use add for quick inserts without ID control.
Use set to control document IDs or overwrite.
add returns a reference to the new doc.
set returns a promise after write completes.

Full Transcript

This visual execution shows how Firebase Firestore adds documents using add and set methods. First, add creates a new document with an auto-generated ID, ensuring unique documents each time. Then, set creates or overwrites a document with a specified ID, allowing control over document keys. The execution table traces each step, showing document IDs and actions. Variable tracking shows how document references and collection contents change. Key moments clarify why add always creates new docs and set can overwrite. The quiz tests understanding of document IDs and method choices. The snapshot summarizes usage rules for add and set.