Querying and filtering in Firebase - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to get a reference to the 'users' collection.

Firebase

const usersRef = firestore.collection([1]);

Drag options to blanks, or click blank then click option'

A'users'

B'user'

C'data'

D'accounts'

Attempts:

3 left

💡 Hint

Common Mistakes

Using singular 'user' instead of 'users'.

Forgetting quotes around the collection name.

The collection name must be exactly 'users' to reference the correct data.

2fill in blank

medium

Complete the code to filter users where age is greater than 18.

Firebase

const adultUsersQuery = usersRef.where('age', [1], 18);

Drag options to blanks, or click blank then click option'

A==

B<

C>

D<=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '==' which checks for equality, not greater than.

Using '<' which filters younger users.

The '>' operator filters documents where the 'age' field is greater than 18.

3fill in blank

hard

Fix the error in the query to order users by their 'score' in descending order.

Firebase

const topUsersQuery = usersRef.orderBy('score', [1]);

Drag options to blanks, or click blank then click option'

A'ascending'

B'desc'

C'asc'

D'descending'

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'ascending' or 'descending' which are invalid in Firestore.

Using 'asc' which orders from lowest to highest.

The correct string for descending order in Firestore is 'desc'.

4fill in blank

hard

Fill both blanks to create a query that filters users with 'status' equal to 'active' and orders by 'lastLogin' ascending.

Firebase

const activeUsersQuery = usersRef.where('status', [1], [2]).orderBy('lastLogin', 'asc');

Drag options to blanks, or click blank then click option'

A==

B'active'

C'inactive'

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '!=' instead of '==' for equality.

Using 'inactive' instead of 'active' as the filter value.

The 'where' clause uses '==' to check equality with the string 'active'.

5fill in blank

hard

Fill all three blanks to create a query that filters users with age greater than 21, status equal to 'active', and orders by 'signupDate' descending.

Firebase

const filteredUsersQuery = usersRef.where('age', [1], 21).where('status', [2], [3]).orderBy('signupDate', 'desc');

Drag options to blanks, or click blank then click option'

A>

B==

C'active'

D'inactive'

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' instead of '>' for age.

Using '!=' or wrong status value.

Incorrect orderBy direction string.

The query filters users older than 21 with status 'active' and orders by signupDate descending.

Practice

1. What does the where method do in a Firestore query?

easy

A. Limits the number of documents returned

B. Sorts documents in ascending order

C. Filters documents based on a condition

D. Deletes documents matching a condition

Solution

1. Step 1: Understand the purpose of where

   The where method is used to filter documents by specifying a condition on a field.

2. Step 2: Compare with other query methods

   orderBy sorts documents, limit restricts count, and deletion is unrelated.

3. Final Answer:

   Filters documents based on a condition -> Option C

4. Quick Check:

   where = filter condition [OK]

Hint: Remember: where filters, orderBy sorts, limit restricts [OK]

Common Mistakes:

• Confusing where with orderBy
• Thinking where limits results
• Assuming where deletes data

2. Which of the following is the correct syntax to filter documents where the field age is greater than 18 in Firestore?

easy

A. query.where('age', '>', 18)

B. query.where('age' > 18)

C. query.where('age', 'greater', 18)

D. query.where('age', '>=', '18')

Solution

1. Step 1: Check Firestore where syntax

   The correct syntax uses three arguments: field name, operator as string, and value.

2. Step 2: Validate each option

   query.where('age', '>', 18) matches correct syntax. query.where('age' > 18) misses quotes and arguments. query.where('age', 'greater', 18) uses invalid operator. query.where('age', '>=', '18') compares number to string.

3. Final Answer:

   query.where('age', '>', 18) -> Option A

4. Quick Check:

   where(field, operator, value) correct format [OK]

Hint: Use quotes around operator and field name [OK]

Common Mistakes:

• Omitting quotes around operator
• Using invalid operators like 'greater'
• Comparing number to string

3. Given this Firestore query code:

const query = collection.where('score', '>=', 50).orderBy('score').limit(3);

What will this query return?

medium

A. Top 3 documents with score less than 50

B. 3 documents with the lowest scores 50 or above

C. 3 documents with the highest scores 50 or above

D. All documents with score exactly 50

Solution

1. Step 1: Analyze the where condition

   The query filters documents with score greater than or equal to 50.

2. Step 2: Understand orderBy and limit

   Ordering by score defaults to ascending order, so lowest scores come first. Limit(3) returns first 3 documents in that order.

3. Final Answer:

   3 documents with the lowest scores 50 or above -> Option B

4. Quick Check:

   where >= 50 + orderBy ascending + limit 3 = lowest 3 scores ≥ 50 [OK]

Hint: orderBy defaults ascending unless specified [OK]

Common Mistakes:

• Assuming orderBy sorts descending by default
• Ignoring the where filter
• Thinking limit returns highest scores

4. You wrote this Firestore query:

collection.where('status', '==', 'active').orderBy('createdAt')

But it throws an error about missing index. What is the best fix?

medium

A. Add a composite index for status and createdAt

B. Remove the orderBy clause

C. Change where to filter

D. Use limit(1) to avoid indexing

Solution

1. Step 1: Understand Firestore indexing rules

   Queries with multiple conditions on different fields require composite indexes.

2. Step 2: Identify the error cause

   The query filters by status and orders by createdAt, needing a composite index.

3. Final Answer:

   Add a composite index for status and createdAt -> Option A

4. Quick Check:

   Composite index needed for where + orderBy [OK]

Hint: Create composite index for multiple fields in query [OK]

Common Mistakes:

• Removing orderBy instead of indexing
• Using invalid method 'filter'
• Thinking limit avoids index errors

5. You want to find users who are either under 18 or have a premium subscription. Which Firestore query approach is correct?

hard

A. Use a single query with where('age', '<', 18).orWhere('subscription', '==', 'premium')

B. Use orderBy on age and subscription fields

C. Use whereIn with both conditions combined

D. Use two separate queries with where and combine results in code

Solution

1. Step 1: Understand Firestore query limitations

   Firestore does not support OR queries directly in a single query.

2. Step 2: Choose correct workaround

   Run two queries separately: one for age < 18, another for subscription == premium, then merge results in code.

3. Final Answer:

   Use two separate queries with where and combine results in code -> Option D

4. Quick Check:

   Firestore lacks OR; combine queries manually [OK]

Hint: Firestore OR queries need multiple queries combined [OK]

Common Mistakes:

• Trying to use non-existent orWhere method
• Using whereIn incorrectly for OR logic
• Assuming orderBy filters data