org.apache.beam.sdk.schemas.transforms

Class Select<T>

java.lang.Object

org.apache.beam.sdk.transforms.PTransform<PCollection<T>,PCollection<Row>>

org.apache.beam.sdk.schemas.transforms.Select<T>

All Implemented Interfaces:

java.io.Serializable, HasDisplayData

@Experimental(value=SCHEMAS)
public class Select<T>
extends PTransform<PCollection<T>,PCollection<Row>>

A PTransform for selecting a subset of fields from a schema type.

This transforms allows projecting out a subset of fields from a schema type. The output of this transform is of type Row, though that can be converted into any other type with matching schema using the Convert transform.

For example, consider the following POJO type:

 @DefaultSchema(JavaFieldSchema.class)
 public class UserEvent {
   public String userId;
   public String eventId;
   public int eventType;
   public Location location;
 }

 @DefaultSchema(JavaFieldSchema.class)
 public class Location {
   public double latitude;
   public double longtitude;
 }

Say you want to select just the set of userId, eventId pairs from each element, you would write the following:

 PCollection<UserEvent> events = readUserEvents();
 PCollection<Row> rows = event.apply(Select.fieldNames("userId", "eventId"));
 

It's possible to select a nested field as well. For example, if you want just the location information from each element:

 PCollection<UserEvent> events = readUserEvents();
 PCollection<Location> rows = event.apply(Select.fieldNames("location")
                              .apply(Convert.to(Location.class));
 

See Also:

Serialized Form

Field Summary

Fields inherited from class org.apache.beam.sdk.transforms.PTransform

name

Method Summary

Modifier and Type	Method and Description
PCollection<Row>	expand(PCollection<T> input) Override this method to specify how this PTransform should be expanded on the given InputT.
static <T> Select<T>	fieldAccess(FieldAccessDescriptor fieldAccessDescriptor) Select a set of fields described in a FieldAccessDescriptor.
static <T> Select<T>	fieldIds(java.lang.Integer... ids) Select a set of top-level field ids from the row.
static <T> Select<T>	fieldNames(java.lang.String... names) Select a set of top-level field names from the row.

Methods inherited from class org.apache.beam.sdk.transforms.PTransform

compose, compose, getAdditionalInputs, getDefaultOutputCoder, getDefaultOutputCoder, getDefaultOutputCoder, getKindString, getName, populateDisplayData, toString, validate

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

Method Detail

fieldIds

public static <T> Select<T> fieldIds(java.lang.Integer... ids)

Select a set of top-level field ids from the row.

fieldNames

public static <T> Select<T> fieldNames(java.lang.String... names)

Select a set of top-level field names from the row.

fieldAccess

public static <T> Select<T> fieldAccess(FieldAccessDescriptor fieldAccessDescriptor)

Select a set of fields described in a FieldAccessDescriptor.

This allows for nested fields to be selected as well.

expand

public PCollection<Row> expand(PCollection<T> input)

Description copied from class: PTransform

Override this method to specify how this PTransform should be expanded on the given InputT.

NOTE: This method should not be called directly. Instead apply the PTransform should be applied to the InputT using the apply method.

Composite transforms, which are defined in terms of other transforms, should return the output of one of the composed transforms. Non-composite transforms, which do not apply any transforms internally, should return a new unbound output and register evaluators (via backend-specific registration methods).

Specified by:

expand in class PTransform<PCollection<T>,PCollection<Row>>