Working with Projection

1. Including Fields

Form	Effect
`{name: 1, email: 1}`	Include + _id by default
`{name: 1, _id: 0}`	Exclude _id explicitly
Rule	Cannot mix include/exclude (except _id)

Form	Effect
`{password: 0}`	Exclude; everything else included
`{password: 0, "internal.secret": 0}`	Nested exclusion

Operator	Effect
`$slice: n`	First n (or last if negative)
`$slice: [skip, limit]`	Window
`$elemMatch`	First element matching condition
`$`	First element matching query filter

db.posts.find({}, { comments: { $slice: -3 } });  // last 3 comments

Aspect	Detail
Returns	First array element matching condition
Differs from $	Condition specified in projection (not query)

db.classes.find({}, { students: { $elemMatch: { grade: { $gte: 90 } } } });

Aspect	Detail
Syntax	`{ "items.$": 1 }`
Returns	Only the first matching element from query filter
Limitation	Only one positional projection per query

Form	Effect
`{_id: 0, name: 1}`	Only valid mix of include/exclude
Use case	Cleaner export, joining external systems

Field	Returns
`{$meta: "textScore"}`	Relevance score (requires $text query)
`{$meta: "indexKey"}`	Index key used (debugging)
`{$meta: "searchScore"}`	Atlas Search score
`{$meta: "searchHighlights"}`	Atlas Search highlights

Form	Effect
`{"profile.city": 1}`	Dot notation include
`{profile: {city: 1}}`	Nested object form
Aggregation	Use `$project` with expressions for renaming

Limitation	Detail
No mix include/exclude	Except _id
One $ per query	Multiple positional updates need filtered ($[id])
Field renaming	Not in find(); use aggregation $project
Computed fields	Aggregation only (find supports limited 4.4+ expressions)

Benefit	How
Network bytes ↓	Exclude large fields (images, blobs)
Covered query	All projected fields in index → no doc fetch
CPU ↓	Less BSON serialization
Memory ↓	Smaller client-side objects

Note: A covered query requires projection that includes ONLY indexed fields and excludes _id (unless _id is part of the index).