Finding and querying documents in Express - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to find all documents in a MongoDB collection using Express.

Express

app.get('/items', async (req, res) => {
  const items = await Item.[1]();
  res.json(items);
});

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

Afind

Binsert

Cupdate

Ddelete

Attempts:

3 left

💡 Hint

Common Mistakes

Using insert instead of find

Using update or delete which modify data

The find method retrieves all documents from the collection.

2fill in blank

medium

Complete the code to find documents matching a condition in Express.

Express

app.get('/users', async (req, res) => {
  const users = await User.find({ age: { [1]: 18 } });
  res.json(users);
});

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

A$lt

B$ne

C$gte

D$in

Attempts:

3 left

💡 Hint

Common Mistakes

Using $lt which means less than

Using $ne which means not equal

The $gte operator finds documents where age is greater than or equal to 18.

3fill in blank

hard

Fix the error in the code to find a single document by ID.

Express

app.get('/post/:id', async (req, res) => {
  const post = await Post.[1](req.params.id);
  res.json(post);
});

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

AfindOneById

BfindAllById

CfindByID

DfindById

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect method names like findOneById or findByID

Using findAllById which does not exist

The correct method to find a document by its ID is findById.

4fill in blank

hard

Fill both blanks to find documents where status is 'active' and sort by createdAt descending.

Express

const activeItems = await Item.find({ status: [1] }).[2]({ createdAt: -1 });

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

A'active'

Bsort

Cfilter

D'inactive'

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'inactive' instead of 'active'

Using 'filter' instead of 'sort' for ordering

We filter by status 'active' and then sort by createdAt descending using sort.

5fill in blank

hard

Fill all three blanks to find users older than 21, select only name and email fields, and limit results to 5.

Express

const users = await User.find({ age: { [1]: 21 } })
  .[2]('name email')
  .[3](5);

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

A$gt

Bselect

Climit

Dskip

Attempts:

3 left

💡 Hint

Common Mistakes

Using $gte instead of $gt

Using skip instead of limit

Not selecting fields properly

Use $gt to find age greater than 21, select to choose fields, and limit to restrict number of results.

Practice

1. In Express with Mongoose, which method is used to find all documents matching a condition in a collection?

easy

A. Model.delete()

B. Model.create()

C. Model.update()

D. Model.find()

Solution

1. Step 1: Understand the purpose of Model.find()

   This method retrieves documents from the database that match the given query conditions.

2. Step 2: Differentiate from other methods

   Model.create() adds new documents, Model.update() modifies existing ones, and Model.delete() removes documents.

3. Final Answer:

   Model.find() -> Option D

4. Quick Check:

   Find documents = Model.find() [OK]

Hint: Use find() to get matching documents [OK]

Common Mistakes:

• Confusing find() with create()
• Using update() to retrieve documents
• Trying to delete documents to find them

2. Which of the following is the correct syntax to find all users with age 30 using Mongoose in Express?

easy

A. User.find({ age: 30 })

B. User.find(age == 30)

C. User.find('age: 30')

D. User.find({ 'age' = 30 })

Solution

1. Step 1: Use an object to specify query conditions

   The query must be an object with key-value pairs, like { age: 30 }.

2. Step 2: Check syntax correctness

   User.find({ age: 30 }) uses correct JavaScript object syntax. Options B, C, and D have syntax errors or wrong formats.

3. Final Answer:

   User.find({ age: 30 }) -> Option A

4. Quick Check:

   Query object syntax = User.find({ age: 30 }) [OK]

Hint: Use curly braces with key: value for queries [OK]

Common Mistakes:

• Using comparison operators inside find()
• Passing query as a string
• Using assignment (=) instead of colon (:) in object

3. What will be the output of this code snippet in Express using Mongoose?

const users = await User.find({ active: true }, 'name email');
console.log(users);

medium

A. An array of user objects with only 'name' and 'email' fields

B. An array of user objects with all fields including 'active'

C. A single user object with 'name' and 'email' fields

D. An error because projection syntax is wrong

Solution

1. Step 1: Understand the query parameters

   The first argument is the filter { active: true }, so only active users are returned.

2. Step 2: Understand the projection string

   The second argument 'name email' selects only these fields to be included in the returned documents.

3. Final Answer:

   An array of user objects with only 'name' and 'email' fields -> Option A

4. Quick Check:

   Projection limits fields = An array of user objects with only 'name' and 'email' fields [OK]

Hint: Second argument in find() selects fields to return [OK]

Common Mistakes:

• Expecting all fields to be returned
• Thinking projection returns a single object
• Using wrong projection syntax causing errors

4. Identify the error in this Mongoose query:

const results = await Product.find({ price: { $gt: 100 } }, { name, price });

medium

A. The find method cannot use async/await

B. The query operator $gt is invalid

C. The projection object is missing quotes around field names

D. The filter object should be a string

Solution

1. Step 1: Check the projection argument

   The projection should be an object with field names as strings or keys, like { name: 1, price: 1 }.

2. Step 2: Identify the syntax error

   Using { name, price } without values or quotes is invalid JavaScript object syntax here.

3. Final Answer:

   The projection object is missing quotes around field names -> Option C

4. Quick Check:

   Projection keys must be strings or key-value pairs [OK]

Hint: Projection keys need explicit values or quotes [OK]

Common Mistakes:

• Using shorthand object keys without values in projection
• Misunderstanding $gt operator
• Thinking async/await is invalid with find()

5. You want to find all orders where the status is 'shipped' and the total is greater than 50, but only return the order ID and total fields. Which is the correct Mongoose query in Express?

hard

A. Order.find({ status: 'shipped', total: { $gt: 50 } }, { _id: 1, total: 1 })

B. Order.find({ status: 'shipped', total: { $gt: 50 } }, '_id total')

C. Order.find({ status: 'shipped', total: { $gt: 50 } }, { _id: true, total: true })

D. Order.find({ status: 'shipped' && total > 50 }, '_id total')

Solution

1. Step 1: Write the correct filter object

   The filter must be { status: 'shipped', total: { $gt: 50 } } to find orders with status 'shipped' and total greater than 50.

2. Step 2: Use correct projection syntax

   Passing a string with space-separated field names like '_id total' is valid for projection in Mongoose.

3. Step 3: Evaluate options

   Order.find({ status: 'shipped', total: { $gt: 50 } }, { _id: 1, total: 1 }) uses an object projection with 1s which is valid but the question asks for the best correct query. Order.find({ status: 'shipped', total: { $gt: 50 } }, '_id total') uses string projection which is simpler and correct. Order.find({ status: 'shipped' && total > 50 }, '_id total') has invalid filter syntax. Order.find({ status: 'shipped', total: { $gt: 50 } }, { _id: true, total: true }) uses boolean true instead of 1 which is invalid in Mongoose projection.

4. Final Answer:

   Order.find({ status: 'shipped', total: { $gt: 50 } }, '_id total') -> Option B

5. Quick Check:

   Filter object + string projection = Order.find({ status: 'shipped', total: { $gt: 50 } }, '_id total') [OK]

Hint: Use object for filter, string for projection fields [OK]

Common Mistakes:

• Using logical operators incorrectly in filter
• Using boolean true instead of 1 in projection
• Passing projection as object with invalid values