JPA Criteria API group_concat usage - jpa

Question

I am currently working on a report which needs a group_concat for one of the fields.
CriteriaQuery<GameDetailsDto> criteriaQuery = criteriaBuilder
.createQuery(GameDetailsDto.class);
Root<BetDetails> betDetails = criteriaQuery.from(BetDetails.class);
Expression<String> betSelection = betDetails.get("winningOutcome");
criteriaQuery.multiselect(
// other fields to select
criteriaBuilder.function("group_concat", String.class, betSelection),
// other fields to select
);
//predicate, where clause and other filters
TypedQuery<GameDetailsDto> typedQuery = entityManager.createQuery(criteriaQuery);
this throws a null pointer exception on the line:
TypedQuery<GameDetailsDto> typedQuery = entityManager.createQuery(criteriaQuery);
did i incorrectly use the function method of the criteriaBuilder?
the documentations says:
function(String name, Class<T> type, Expression<?>... args);

Answer 1

I figured out how to do this with Hibernate-jpa-mysql:
1.) created a GroupConcatFunction class extending org.hibernate.dialect.function.SQLFunction (this is for single column group_concat for now)
public class GroupConcatFunction implements SQLFunction {
#Override
public boolean hasArguments() {
return true;
}
#Override
public boolean hasParenthesesIfNoArguments() {
return true;
}
#Override
public Type getReturnType(Type firstArgumentType, Mapping mapping)
throws QueryException {
return StandardBasicTypes.STRING;
}
#Override
public String render(Type firstArgumentType, List arguments,
SessionFactoryImplementor factory) throws QueryException {
if (arguments.size() != 1) {
throw new QueryException(new IllegalArgumentException(
"group_concat shoudl have one arg"));
}
return "group_concat(" + arguments.get(0) + ")";
}
}
2.) i created the CustomMySql5Dialect class extending org.hibernate.dialect.MySQL5Dialect and registered the group_concat class created in step 1
3.) On the app context, i updated the jpaVendorAdapter to use the CustomMySql5Dialect as the databasePlatform
4.) Finally to use it
criteriaBuilder.function("group_concat", String.class,
sampleRoot.get("sampleColumnName"))

Answer 2

Simple solution: instead of creating the whole class, just use SQLFunctionTemplate.
new SQLFunctionTemplate(StandardBasicTypes.STRING, "group_concat(?1)")
and then register this function in your own SQL dialect (eg. in constructor)
public class MyOwnSQLDialect extends MySQL5Dialect {
public MyOwnSQLDialect() {
super();
this.registerFunction("group_concat", new SQLFunctionTemplate(StandardBasicTypes.STRING, "group_concat(?1)"));
}
}

Answer 3

Suggested property:
spring.jpa.properties.hibernate.metadata_builder_contributor = com.inn.core.generic.utils.SqlFunctionsMetadataBuilderContributor
and class:
import org.hibernate.boot.MetadataBuilder;
import org.hibernate.boot.spi.MetadataBuilderContributor;
import org.hibernate.dialect.function.StandardSQLFunction;
import org.hibernate.type.StandardBasicTypes;
import org.springframework.stereotype.Component;
#Component
public class SqlFunctionsMetadataBuilderContributor implements MetadataBuilderContributor {
#Override
public void contribute(MetadataBuilder metadataBuilder) {
metadataBuilder.applySqlFunction("config_json_extract",
new StandardSQLFunction("json_extract", StandardBasicTypes.STRING));
metadataBuilder.applySqlFunction("JSON_UNQUOTE",
new StandardSQLFunction("JSON_UNQUOTE", StandardBasicTypes.STRING));
metadataBuilder.applySqlFunction("group_concat",
new StandardSQLFunction("group_concat", StandardBasicTypes.STRING));
}
}