Deleting fields in Firebase - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does deleting a field in Firebase Firestore mean?

Deleting a field means removing a specific piece of data from a document without deleting the whole document.

beginner

Which Firebase method is used to delete a field in a Firestore document?

The method is FieldValue.delete(), used inside an update operation to remove a field.

beginner

How do you delete a field named age from a Firestore document in JavaScript?

Use update(docRef, { age: FieldValue.delete() }) to remove the age field.

intermediate

What happens if you try to delete a field that does not exist in a Firestore document?

Nothing changes; Firestore ignores the delete request for a non-existing field without error.

beginner

Why might you prefer deleting a field over deleting the entire document in Firestore?

Deleting a field keeps the document and other data intact, useful when only part of the data needs removal.

Which Firebase function deletes a field from a Firestore document?

A FieldValue.delete()

B deleteField()

C removeField()

D FieldValue.remove()

The correct method to delete a field is FieldValue.delete(), used in update operations.

What happens if you delete a field that does not exist in Firestore?

A Nothing changes

B The document is deleted

C An error is thrown

D The field is created and then deleted

Firestore ignores delete requests for non-existing fields without error or changes.

To delete a field named 'status' in Firestore, which code snippet is correct?

A docRef.deleteField('status')

B deleteField(docRef, 'status')

C remove(docRef.status)

D update(docRef, { status: FieldValue.delete() })

Use update with FieldValue.delete() to remove a specific field.

Deleting a field in Firestore affects which of the following?

A All documents in the collection are deleted

B The entire document is removed

C Only the specified field is removed

D The collection is deleted

Only the specified field is removed; the rest of the document stays intact.

Why is deleting a field useful instead of deleting the whole document?

A It archives the document

B It saves bandwidth and keeps other data

C It deletes the entire collection

D It backs up the document automatically

Deleting a field removes only that data, preserving the rest and saving resources.

Explain how to delete a field from a Firestore document and why you might want to do this.

Think about removing just one piece of data without deleting everything.

What happens if you try to delete a field that does not exist in a Firestore document?

Consider how Firestore handles requests for missing data.

Practice

1. What does the deleteField() function do in Firebase Firestore?

easy

A. It deletes the entire document.

B. It removes a specific field from a document.

C. It adds a new field to a document.

D. It updates the value of a field.

Solution

1. Step 1: Understand the purpose of deleteField()

   This function is designed to remove a specific field from a Firestore document without deleting the whole document.

2. Step 2: Differentiate from other operations

   Deleting a document removes all data, adding or updating changes or adds fields, but deleteField() only removes one field.

3. Final Answer:

   It removes a specific field from a document. -> Option B

4. Quick Check:

   deleteField() removes fields = A [OK]

Hint: Remember: deleteField() targets one field, not whole document [OK]

Common Mistakes:

• Confusing field deletion with document deletion
• Thinking deleteField() adds or updates fields
• Assuming deleteField() works on collections

2. Which of the following is the correct syntax to delete a field named age from a Firestore document using updateDoc?

easy

A. updateDoc(docRef, deleteField('age'));

B. updateDoc(docRef, { deleteField: 'age' });

C. updateDoc(docRef, { age: null });

D. updateDoc(docRef, { age: deleteField() });

Solution

1. Step 1: Recall the correct usage of deleteField()

   The function is used as a value in the update object for the field to delete, like { fieldName: deleteField() }.

2. Step 2: Check each option's syntax

   updateDoc(docRef, { age: deleteField() }); correctly uses age: deleteField(). updateDoc(docRef, { deleteField: 'age' }); wrongly uses the function name as a key. updateDoc(docRef, { age: null }); sets the field to null, which does not delete it. updateDoc(docRef, deleteField('age')); incorrectly passes the field name as an argument to deleteField().

3. Final Answer:

   updateDoc(docRef, { age: deleteField() }); -> Option D

4. Quick Check:

   Use field: deleteField() inside updateDoc = A [OK]

Hint: Use field name as key and deleteField() as value [OK]

Common Mistakes:

• Passing field name as argument to deleteField()
• Setting field to null instead of deleting
• Using deleteField as a key instead of a function

3. Given the following code snippet, what will be the content of the document after execution?

const docRef = doc(db, 'users', 'user1');
await updateDoc(docRef, { city: deleteField() });

Assuming the document initially has fields: { name: 'Anna', city: 'Paris', age: 30 }

medium

A. { name: 'Anna', city: 'Paris', age: 30 }

B. { name: 'Anna', city: null, age: 30 }

C. { name: 'Anna', age: 30 }

D. Document deleted

Solution

1. Step 1: Understand what deleteField() does in updateDoc

   It removes the specified field from the document, so city will be removed.

2. Step 2: Analyze the document after update

   Original fields are name, city, and age. After deletion, city is gone, leaving name and age.

3. Final Answer:

   { name: 'Anna', age: 30 } -> Option C

4. Quick Check:

   deleteField() removes field = { name, age } [OK]

Hint: Deleted fields disappear completely, not set to null [OK]

Common Mistakes:

• Thinking deleted fields become null
• Assuming document is deleted
• Believing field value remains unchanged

4. You wrote this code to delete the field status from a Firestore document:

await updateDoc(docRef, { deleteField('status') });

Why does this code cause an error?

medium

A. Because deleteField must be used as a value, not as a key.

B. Because updateDoc cannot delete fields.

C. Because deleteField requires the document reference as argument.

D. Because status field does not exist.

Solution

1. Step 1: Identify the syntax error in the update object

   The object passed to updateDoc must have keys as field names and values as the new values or deleteField(). Here, deleteField('status') is used as a key, which is invalid.

2. Step 2: Correct usage of deleteField()

   The correct syntax is { status: deleteField() }, where status is the key and deleteField() is the value.

3. Final Answer:

   Because deleteField must be used as a value, not as a key. -> Option A

4. Quick Check:

   Field name as key, deleteField() as value = D [OK]

Hint: Use field name as key, deleteField() as value in updateDoc [OK]

Common Mistakes:

• Using deleteField() as key instead of value
• Passing arguments to deleteField()
• Assuming updateDoc can't delete fields

5. You want to remove multiple fields phone and address from a Firestore document in a single update. Which code snippet correctly deletes both fields?

hard

A. await updateDoc(docRef, { phone: deleteField(), address: deleteField() });

B. await updateDoc(docRef, { deleteField('phone'), deleteField('address') });

C. await updateDoc(docRef, { phone: null, address: null });

D. await updateDoc(docRef, deleteField(['phone', 'address']));

Solution

1. Step 1: Understand how to delete multiple fields

   Each field to delete must be a key in the update object with deleteField() as its value.

2. Step 2: Evaluate each option

   await updateDoc(docRef, { phone: deleteField(), address: deleteField() }); correctly deletes both fields by specifying them separately. await updateDoc(docRef, { deleteField('phone'), deleteField('address') }); uses invalid syntax with multiple deleteField calls as keys. await updateDoc(docRef, { phone: null, address: null }); sets fields to null but does not delete them. await updateDoc(docRef, deleteField(['phone', 'address'])); incorrectly passes an array to deleteField, which is not supported.

3. Final Answer:

   await updateDoc(docRef, { phone: deleteField(), address: deleteField() }); -> Option A

4. Quick Check:

   Delete multiple fields by listing each with deleteField() = B [OK]

Hint: List each field with deleteField() in updateDoc object [OK]

Common Mistakes:

• Trying to delete multiple fields with one deleteField call
• Setting fields to null instead of deleting
• Using deleteField as keys or passing arrays