Ordering results in Firebase - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to order documents by the field 'score' in ascending order.

Firebase

db.collection('games').orderBy('[1]').get()

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

Aname

Bdate

Cscore

Dlevel

Attempts:

3 left

💡 Hint

Common Mistakes

Using a field name that does not exist in the documents.

Forgetting to call orderBy before get().

The orderBy method sorts documents by the specified field. Here, 'score' is the field to order by.

2fill in blank

medium

Complete the code to order documents by 'date' in descending order.

Firebase

db.collection('games').orderBy('date', '[1]').get()

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

Adesc

Bascending

Cdescending

Dasc

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'ascending' or 'descending' instead of 'asc' or 'desc'.

Omitting the direction argument defaults to ascending.

The second argument to orderBy specifies the direction. Use 'desc' for descending order.

3fill in blank

hard

Fix the error in the code to order by 'level' ascending.

Firebase

db.collection('games').orderBy([1]).get()

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

A'level', 'asc'

B'level', 'desc'

Clevel

D'level'

Attempts:

3 left

💡 Hint

Common Mistakes

Passing multiple arguments inside one parameter.

Not using quotes around the field name.

The orderBy method takes the field name as a string. To order ascending, just provide the field name.

4fill in blank

hard

Fill both blanks to order by 'score' descending and then by 'date' ascending.

Firebase

db.collection('games').orderBy('[1]', '[2]').orderBy('date', 'asc').get()

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

Ascore

Bdesc

Casc

Ddate

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping the order of fields.

Using wrong direction strings.

The first orderBy orders by 'score' descending. The second orders by 'date' ascending.

5fill in blank

hard

Fill all three blanks to order by 'level' descending, then by 'score' ascending, and finally by 'date' ascending.

Firebase

db.collection('games').orderBy('[1]', '[2]').orderBy('[3]', 'asc').orderBy('date', 'asc').get()

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

Alevel

Bdesc

Cscore

Ddate

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up field names and directions.

Using wrong direction strings.

Not chaining orderBy calls correctly.

The first orderBy sorts by 'level' descending. The second orderBy sorts by 'score' ascending. The third orderBy sorts by 'date' ascending.

Practice

1. What does the orderBy method do in Firebase Firestore queries?

easy

A. It deletes documents from the collection.

B. It filters documents based on a condition.

C. It sorts the query results by a specified field.

D. It adds new documents to the collection.

Solution

1. Step 1: Understand the purpose of orderBy

   The orderBy method is used to sort documents in a query result by a specific field.

2. Step 2: Differentiate from other query methods

   Filtering uses where, deleting uses delete, and adding uses add. These are different from sorting.

3. Final Answer:

   It sorts the query results by a specified field. -> Option C

4. Quick Check:

   Ordering = Sorting [OK]

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

Common Mistakes:

• Confusing orderBy with filtering
• Thinking orderBy adds or deletes data
• Assuming orderBy changes data, not just order

2. Which of the following is the correct syntax to order Firestore documents by the field age in descending order?

easy

A. db.collection('users').orderBy('age', 'desc')

B. db.collection('users').orderBy('age', descending)

C. db.collection('users').orderBy('age', 'descending')

D. db.collection('users').orderBy('age', -1)

Solution

1. Step 1: Recall Firestore orderBy syntax

   The correct syntax uses the string 'desc' for descending order, not 'descending' or other variants.

2. Step 2: Check each option

   db.collection('users').orderBy('age', 'desc') uses 'desc' correctly. Options A and B use invalid strings. db.collection('users').orderBy('age', -1) uses a number which is not valid in Firestore.

3. Final Answer:

   db.collection('users').orderBy('age', 'desc') -> Option A

4. Quick Check:

   Descending order = 'desc' string [OK]

Hint: Use 'asc' or 'desc' strings exactly in orderBy [OK]

Common Mistakes:

• Using 'descending' instead of 'desc'
• Using boolean or numbers instead of strings
• Omitting quotes around 'desc'

3. Given this Firestore query:

db.collection('products').orderBy('price', 'asc').limit(3).get()

Which products will be returned?

medium

A. The 3 products with the lowest price.

B. The 3 products with the highest price.

C. All products sorted by price ascending.

D. 3 random products from the collection.

Solution

1. Step 1: Analyze the query order and limit

   The query orders products by price ascending (lowest to highest) and limits results to 3 documents.

2. Step 2: Understand the effect of limit

   Limit(3) returns only the first 3 documents after sorting, so the 3 cheapest products.

3. Final Answer:

   The 3 products with the lowest price. -> Option A

4. Quick Check:

   orderBy ascending + limit 3 = lowest 3 [OK]

Hint: orderBy asc + limit = smallest values first [OK]

Common Mistakes:

• Confusing ascending with descending
• Ignoring the limit clause
• Assuming random order without sorting

4. You wrote this Firestore query:

db.collection('orders').orderBy('date').orderBy('total', 'desc').get()

But it throws an error about missing indexes. What should you do?

medium

A. Use where() instead of orderBy().

B. Create a composite index on 'date' ascending and 'total' descending.

C. Change 'desc' to 'descending' in orderBy.

D. Remove one orderBy to avoid multiple ordering.

Solution

1. Step 1: Understand Firestore composite index requirement

   Ordering by multiple fields requires a composite index matching the order and directions.

2. Step 2: Identify correct index to create

   You must create a composite index on 'date' ascending and 'total' descending to support this query.

3. Final Answer:

   Create a composite index on 'date' ascending and 'total' descending. -> Option B

4. Quick Check:

   Multiple orderBy needs composite index [OK]

Hint: Multiple orderBy? Create matching composite index [OK]

Common Mistakes:

• Ignoring index creation errors
• Using wrong direction strings
• Trying to avoid multiple orderBy instead of indexing

5. You want to get all users ordered by their score descending, but only those who have score greater than 50. Which query is correct?

hard

A. db.collection('users').orderBy('score', 'asc').where('score', '>', 50).get()

B. db.collection('users').orderBy('score').where('score', '>', 50).get()

C. db.collection('users').where('score', '>', 50).orderBy('score').get()

D. db.collection('users').where('score', '>', 50).orderBy('score', 'desc').get()

Solution

1. Step 1: Understand Firestore query rules for inequality filters

   Range filters like '>' require orderBy on the same field as the first ordering field. Use 'desc' for descending order.

2. Step 2: Check each option

   A uses orderBy('score', 'asc'): ascending, not descending.
   B: where('score', '>', 50).orderBy('score', 'desc'): correct for filtered descending order.
   C: orderBy('score') defaults to 'asc'.
   D: orderBy('score') defaults to 'asc'.

3. Final Answer:

   db.collection('users').where('score', '>', 50).orderBy('score', 'desc').get() -> Option D

4. Quick Check:

   Range filter + first orderBy on field + 'desc' [OK]

Hint: Range filters require orderBy on same field and direction [OK]

Common Mistakes:

• Mismatching orderBy direction with where filter
• Omitting orderBy when using range filters
• Using orderBy without specifying direction