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Read Data from MongoDB

Specify Fields To Return

On this page

  • Overview
  • Sample Data
  • Projection Types
  • Specify Fields to Include
  • Exclude the _id Field
  • Additional Information
  • API Documentation

Overview

In this guide, you can learn how to specify which fields to return from a read operation by using a projection. A projection is a document that specifies which fields MongoDB returns from a query.

Sample Data

The examples in this guide use the restaurants collection in the sample_restaurants database from the Atlas sample datasets. To learn how to create a free MongoDB Atlas cluster and load the sample datasets, see the Get Started with Atlas guide.

The documents in this collection are modeled by the following Kotlin data class:

data class Restaurant(
    @BsonId
    val id: ObjectId? = null,
    val name: String,
    val borough: String,
    val cuisine: String
)

Projection Types

You can use a projection to specify which fields to include in a return document, or to specify which fields to exclude.

When specifying certain fields to include in a projection, all other fields are implicitly excluded (except the _id field, which is included by default). You cannot combine inclusion and exclusion statements in a single projection, unless you are excluding the _id field.

To remove the _id field from the returned document, you must explicitly exclude it.

Specify Fields to Include

Use the following syntax to specify the fields to include in the result:

val projection = Projection.fields(
    Projections.include(<fieldName1>, <fieldName2>, ...)
)

The following example uses the find() method to find all restaurants with the name field value of "Emerald Pub". It then uses a projection to return only the name, cuisine, and borough fields of the returned documents.

val projection = Projections.fields(
    Projections.include(
        Restaurant::name.name,
        Restaurant::cuisine.name,
        Restaurant::borough.name
    )
)
val results = collection
    .find(eq(Restaurant::name.name, "Emerald Pub"))
    .projection(projection)
results.forEach { result ->
    println(result)
}
Restaurant(id=5eb3d668b31de5d588f429e2, name=Emerald Pub, borough=Manhattan, cuisine=American)
Restaurant(id=5eb3d668b31de5d588f432dd, name=Emerald Pub, borough=Queens, cuisine=American)

Exclude the _id Field

When specifying fields to include, you can also exclude the _id field from the returned document.

The following example runs the same query as the preceding example, but excludes the _id field from the projection:

val projection = Projections.fields(
    Projections.excludeId(),
    Projections.include(
        Restaurant::name.name,
        Restaurant::cuisine.name,
        Restaurant::borough.name
    )
)
val results = collection
    .find(eq(Restaurant::name.name, "Emerald Pub"))
    .projection(projection)
results.forEach { result ->
    println(result)
}
Restaurant(id=null, name=Emerald Pub, borough=Manhattan, cuisine=American)
Restaurant(id=null, name=Emerald Pub, borough=Queens, cuisine=American)

Additional Information

To learn more about projections, see the Project Fields guide in the MongoDB Server Manual.

API Documentation

To learn more about any of the methods or types discussed in this guide, see the following API Documentation:

  • find()

  • projection()

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Specify Documents to Return