ProjectionQuery

Summary

• Index
• Defining Projection Classes
• Filters and Specifications
• Pagination and Sorting
• Executing Queries
• Custom Select Handlers
  • Creating a Custom Select Handler
  • Registering the Custom Select Handler
  • Using the Custom Select Handler in Projections
• Custom Filter Handlers
• Logging and Debug
• For Spring Boot users

Custom Select Handlers

In some cases, you may need to perform custom transformations or calculations on the selected fields in your projection. The ProjectionQuery library allows you to define custom select handlers to achieve this.

To create a custom select handler, you need to implement the ProjectionSelectOperatorHandler interface, which defines how the selected field expression should be constructed. Once you have implemented your custom select handler, you can register it with the ProjectionSelectOperatorProvider and use it in your projections.

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Creating a Custom Select Handler ↑

Here we will create a handler that will select only the numbers from a given property.

public class OnlyNumbersHandler implements ProjectionSelectOperatorHandler {

    @Override
    public boolean aggregate() {
        return false;
    }

    @Override
    public Expression<?> apply(PathResolver pathResolver, CriteriaBuilder cb, Root<?> root, String fieldName) {
        return cb.function(
                "REGEXP_REPLACE",
                String.class,
                pathResolver.resolve(root, fieldName),
                cb.literal("[^0-9]"),
                cb.literal("g")
        );
    }
}

In some scenarios, you may need to perform custom transformations or calculations directly in the SELECT clause of your projection.

Notes:

1. The SQL function used inside the handler must be supported by your underlying database. The example above uses REGEXP_REPLACE, which may not be available in all SQL dialects.
2. If your handler performs an aggregation (e.g., COUNT, SUM, AVG), you must return true in the aggregate() method.

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Registering the Custom Select Handler ↑

After creating your custom select handler, you need to register it with ProjectionSelectOperatorProvider so that it can be used in your projections.

ProjectionSelectOperatorProvider
    .getInstance()
    .register(new OnlyNumbersHandler());

Note: You only need to register the handler once, usually during application initialization.

Using the Custom Select Handler in Projections ↑

After registering your custom select handler, you can use it in your projections. To do this, simply reference the handler class in the @ProjectionField annotation:

@Projection(of = Customer.class)
public record CustomerProjection(
        @ProjectionField Long id,
        @ProjectionField String name,
        @ProjectionField(selectHandler = OnlyNumbersHandler.class) String phoneNumber
) { }

The SQL generated for this projection would look something like this:

select
    id,
    name,
    regexp_replace(phone_number, '[^0-9]', 'g') as phone_number
from customer

← Executing Queries · ↑ Back to top · Next → Custom Filter Handlers

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