Projection for selecting fields in MongoDB - Step-by-Step Execution

Concept Flow - Projection for selecting fields

Start with full documents

↓

Specify fields to include or exclude

↓

MongoDB applies projection

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Return documents with only selected fields

Projection lets you pick which fields to show from documents by including or excluding them.

Execution Sample

MongoDB

db.users.find({}, {name: 1, age: 1, _id: 0})

This query returns only the name and age fields from all user documents, hiding the _id field.

Execution Table

Step	Input Document	Projection Spec	Output Document
1	{"_id": 1, "name": "Alice", "age": 30, "city": "NY"}	{"name": 1, "age": 1, "_id": 0}	{"name": "Alice", "age": 30}
2	{"_id": 2, "name": "Bob", "age": 25, "city": "LA"}	{"name": 1, "age": 1, "_id": 0}	{"name": "Bob", "age": 25}
3	{"_id": 3, "name": "Carol", "age": 27, "city": "Chicago"}	{"name": 1, "age": 1, "_id": 0}	{"name": "Carol", "age": 27}
4	No more documents	N/A	Query ends

💡 All documents processed with specified projection, query ends.

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
Input Document	Full document	{"_id":1,"name":"Alice","age":30,"city":"NY"}	{"_id":2,"name":"Bob","age":25,"city":"LA"}	{"_id":3,"name":"Carol","age":27,"city":"Chicago"}	No more documents
Projection Spec	{"name":1,"age":1,"_id":0}	{"name":1,"age":1,"_id":0}	{"name":1,"age":1,"_id":0}	{"name":1,"age":1,"_id":0}	N/A
Output Document	N/A	{"name":"Alice","age":30}	{"name":"Bob","age":25}	{"name":"Carol","age":27}	Query ends

Key Moments - 3 Insights

Why does the _id field disappear in the output even though it exists in the input?

Because the projection explicitly sets _id to 0 (exclude), MongoDB removes it from the output as shown in execution_table rows 1-3.

Can you mix including and excluding fields in the same projection?

No, except for _id, you must either include fields (set to 1) or exclude fields (set to 0). Here we include name and age, exclude _id, consistent in all rows.

What happens if a field is not mentioned in the projection?

Fields not included in the projection are excluded by default when using inclusion style, so city is missing in output documents as seen in execution_table.

Visual Quiz - 3 Questions

Look at the execution_table at Step 2, what fields are present in the output document?

A name, age, and city

B name and age only

C all fields including _id

D only _id

💡 Hint

Check the Output Document column at Step 2 in execution_table.

At which step does the query finish processing all documents?

A Step 3

B Step 1

C Step 4

D Step 2

💡 Hint

Look for 'No more documents' in Input Document column in execution_table.

If we remove '_id: 0' from the projection, what changes in the output documents?

A _id field will be included in output

B _id field will be excluded

C name and age will be excluded

D All fields will be included

💡 Hint

Refer to key_moments about excluding _id and execution_table output documents.

Concept Snapshot

MongoDB Projection Syntax:
find(query, {field1: 1, field2: 1, _id: 0})
Includes only specified fields, excludes others.
_id is included by default; set _id:0 to hide it.
Cannot mix inclusion and exclusion except for _id.
Projection controls which fields appear in results.

Full Transcript

Projection in MongoDB lets you select which fields to show from documents. You write a query with an empty filter {} to get all documents, then add a projection object like {name:1, age:1, _id:0} to include name and age fields but hide the _id field. MongoDB processes each document, applies the projection, and returns only the selected fields. Fields not included are excluded by default when using inclusion style. You cannot mix including and excluding fields except for the _id field. This example shows three documents with full fields, and after projection, only name and age remain. The _id field disappears because we set it to 0 in the projection. The query ends after all documents are processed.

Practice

1. What does projection do in a MongoDB query?

easy

A. It selects which fields to include or exclude in the query result.

B. It sorts the documents in the collection.

C. It updates the documents in the collection.

D. It deletes documents from the collection.

Solution

1. Step 1: Understand the purpose of projection

   Projection in MongoDB is used to specify which fields to include or exclude in the output of a query.

2. Step 2: Differentiate projection from other operations

   Sorting, updating, and deleting are different operations and not related to projection.

3. Final Answer:

   It selects which fields to include or exclude in the query result. -> Option A

4. Quick Check:

   Projection = Select fields [OK]

Hint: Projection picks fields to show, not to sort or update [OK]

Common Mistakes:

• Confusing projection with sorting
• Thinking projection updates data
• Assuming projection deletes documents

2. Which of the following is the correct syntax to include only the name and age fields in a MongoDB find query?

easy

A. { name: 1, age: 1 }

B. { name: true, age: true }

C. { name: 'include', age: 'include' }

D. { name: 0, age: 0 }

Solution

1. Step 1: Recall projection syntax

   In MongoDB, to include fields, you set them to 1 in the projection document.

2. Step 2: Check each option

   { name: 1, age: 1 } uses 1 correctly; { name: true, age: true } uses true which is valid in MongoDB; { name: 'include', age: 'include' } uses strings which is invalid; { name: 0, age: 0 } excludes fields instead of including.

3. Final Answer:

   { name: 1, age: 1 } -> Option A

4. Quick Check:

   Include fields = 1 [OK]

Hint: Use 1 to include fields, not strings [OK]

Common Mistakes:

• Using strings like 'include'
• Using 0 to include fields

3. Given the collection documents:

{ _id: 1, name: "Alice", age: 25, city: "NY" }

{ _id: 2, name: "Bob", age: 30, city: "LA" }

What will be the result of the query db.collection.find({}, { name: 1, city: 1 })?

medium

A. [{ _id: 1, age: 25 }, { _id: 2, age: 30 }]

B. [{ name: "Alice", city: "NY" }, { name: "Bob", city: "LA" }]

C. [{ _id: 1, name: "Alice", city: "NY" }, { _id: 2, name: "Bob", city: "LA" }]

D. [{ name: "Alice", age: 25, city: "NY" }, { name: "Bob", age: 30, city: "LA" }]

Solution

1. Step 1: Understand default _id behavior

   By default, MongoDB includes the _id field unless explicitly excluded.

2. Step 2: Check projection fields

   The query includes name and city fields with 1, so these fields plus _id will appear.

3. Final Answer:

   [{ _id: 1, name: "Alice", city: "NY" }, { _id: 2, name: "Bob", city: "LA" }] -> Option C

4. Quick Check:

   Projection includes fields + _id by default [OK]

Hint: Projection includes _id unless set to 0 [OK]

Common Mistakes:

• Assuming _id is excluded by default
• Expecting age field in result
• Confusing included and excluded fields

4. What is wrong with this MongoDB query to exclude the password field?

db.users.find({}, { password: 1 })

medium

A. The syntax for projection is incorrect.

B. It includes the password field instead of excluding it.

C. You cannot exclude fields in MongoDB projection.

D. The query will return an error because password is a reserved word.

Solution

1. Step 1: Understand projection values

   Setting a field to 1 includes it; setting to 0 excludes it.

2. Step 2: Analyze the query

   The query sets password: 1, so it includes the password field (and _id), not excludes it.

3. Final Answer:

   It includes the password field instead of excluding it. -> Option B

4. Quick Check:

   Use 0 to exclude fields [OK]

Hint: Use 0 to exclude fields, 1 to include [OK]

Common Mistakes:

• Using 1 to exclude fields
• Thinking password is reserved word
• Believing exclusion is not possible

5. You want to fetch documents from a collection but exclude the _id and password fields while including all others. Which projection is correct?

hard

A. { _id: 1, password: 1 }

B. { _id: 1, password: 0 }

C. { _id: 0, password: 1 }

D. { _id: 0, password: 0 }

Solution

1. Step 1: Understand exclusion in projection

   Setting fields to 0 excludes them from the result.

2. Step 2: Apply exclusion to _id and password

   To exclude both _id and password, set both to 0 in the projection.

3. Final Answer:

   { _id: 0, password: 0 } -> Option D

4. Quick Check:

   Exclude fields with 0, include others by default [OK]

Hint: Set unwanted fields to 0, others stay included [OK]

Common Mistakes:

• Mixing inclusion and exclusion in projection
• Forgetting to exclude _id explicitly
• Setting excluded fields to 1