Spring R2DBC query by example with projected/custom entity - spring-data

Question

The title maybe is not properly written but here is what, more or less, I want to achieve.
I would like to be able to write dynamic queries with use of Query by Example that would join multiple tables and create (projection?) DTO for me.
This DTO would have fields that are mapped to different columns in joined tables. Consider following:
Tables:
CREATE TABLE address
(
id SERIAL,
address_code VARCHAR(255) NOT NULL,
street_name VARCHAR(255),
building_number VARCHAR(255)
);
CREATE TABLE account
(
id SERIAL,
account_number BIGINT UNIQUE
);
CREATE TABLE customer
(
id SERIAL,
name VARCHAR(255)
)
I would like to be able to create a query which result would be:
address.address_code, account.account_number, customer.name
so basically the result would be a custom DTO. I also mentioned that I would like to have this backed up with Query by Example because I will to dynamically append WHERE clauses so I thought that if I created a DTO like:
public record CustomQueryResultDTO(String addressCode, BigInteger accountNumber, String name) {}
I could simply query just like it is in Spring R2DBC documentation.
The problem here is that I am not sure what should be a viable solution for such problem because on one hand I would like to reuse ReactiveQueryByExampleExecutor but that would mean that I have to create something like:
#Repository
public interface CustomQueryResultRepository extends ReactiveCrudRepository<CustomQueryResultDTO, Integer>, ReactiveQueryByExampleExecutor<CustomQueryResultDTO> {
}
Which kind of seems to me not a way to go as I do not have a corresponding table for CustomQueryResultDTO therefore there is really no mapping for this repository interface - or am I overthinking this and it is actually a way to go?

Answer 1

I think you are potentially overthinking it.
You can do it in a number of ways (note Java 17 text blocks):
Via R2DBC JPA-like #Query
Create a normal ReactiveCrudRepository but collect into a projection (DTOP)
// Repository
#Repository
public interface UserRefreshTokenRepository extends ReactiveCrudRepository<UserRefreshToken, Integer> {
#Query(
"""
select *
from user.user_refresh_tokens t
join user.user_infos c on c.user_id = t.user_id
where c.username = :username
"""
)
Flux<UserRefreshTokenDtop> findAllByUsername(String username);
}
// Entity
#Data
#Builder
#AllArgsConstructor
#NoArgsConstructor
#ToString(exclude = {"refreshToken"})
#Table(schema = "user", name = "user_refresh_tokens")
public class UserRefreshToken {
#Id private Integer id;
private String userId;
private String username; # will be joined
private String ipAddr;
private OffsetDateTime createdAt;
private String refreshToken;
private OffsetDateTime refreshTokenIat;
private OffsetDateTime refreshTokenExp;
}
// DTO projection
public interface UserRefreshTokenDtop {
Integer getId();
String getUserId();
String getUsername(); # will be joined
String getIpAddr();
OffsetDateTime getRefreshTokenIat();
OffsetDateTime getRefreshTokenExp();
}
Via DatabaseClient
This one also uses TransactionalOperator to ensure query atomicity
private final DatabaseClient client;
private final TransactionalOperator operator;
#Override
public void deleteAllUsedExpiredAttempts(Duration resetInterval) {
// language=PostgreSQL
String allUsedExpiredAttempts = """
select t.id failed_id, c.id disable_id, t.username
from user.failed_sign_attempts t
join user.disable_sign_attempts c on c.username = t.username
where c.is_used = true
and :now >= c.expires_at + interval '%d seconds'
""";
// POTENTIAL SQL injection - half-arsed but %d ensures that only Number is allowed
client
.sql(String.format(allUsedExpiredAttempts, resetInterval.getSeconds()))
.bind("now", Instant.now())
.fetch()
.all()
.flatMap(this::deleteFailed)
.flatMap(this::deleteDisabled)
.as(operator::transactional)
.subscribe(v1 -> log.debug("Successfully reset {} user(s)", v1));
}
Via R2dbcEntityTemplate
I don't have a working example but it is pain in the ass to join via the .join() operator
If you are interested check the docs for R2dbcEntityTemplate
13.4.3. Fluent API > Methods for the Criteria Class
https://docs.spring.io/spring-data/r2dbc/docs/current/reference/html