Introduction

In this article, you are going to learn how to map a JPA entity to the ResultSet of a SQL query using the @Subselect Hibernate-specific annotation.

List all PostgreSQL functions

Let’s assume we have two PostgreSQL functions in our database:

CREATE OR REPLACE FUNCTION public.count_comments(
    IN postid bigint,
    OUT commentcount bigint)
  RETURNS bigint AS
'     BEGIN         
      SELECT COUNT(*) INTO commentCount         
      FROM post_comment          
      WHERE post_id = postId;     
      END; 
'
  LANGUAGE plpgsql VOLATILE
  COST 100;
ALTER FUNCTION public.count_comments(bigint)
  OWNER TO postgres;

CREATE OR REPLACE FUNCTION public.post_comments(postid bigint)
  RETURNS refcursor AS
'     DECLARE         
           postComments REFCURSOR;     
      BEGIN         
      OPEN postComments FOR              
      SELECT *              
      FROM post_comment               
      WHERE post_id = postId;          
      RETURN postComments;     
      END; 
'
  LANGUAGE plpgsql VOLATILE
  COST 100;
ALTER FUNCTION public.post_comments(bigint)
  OWNER TO postgres;

In our application, we want to know all the PostgreSQL functions we can call, and, for this purpose, we can use the following SQL query:

SELECT 
    functions.routine_name as name,
    string_agg(functions.data_type, ',') as params
FROM (
    SELECT 
        routines.routine_name, 
        parameters.data_type, 
        parameters.ordinal_position
    FROM 
        information_schema.routines
    LEFT JOIN 
        information_schema.parameters 
    ON 
        routines.specific_name = parameters.specific_name
    WHERE 
        routines.specific_schema='public'
    ORDER BY 
        routines.routine_name, 
        parameters.ordinal_position
) AS functions
GROUP BY functions.routine_name

When running the SQL query above, we get the following result set:

That’s great, but we want this result set to be mapped as a JPA entity, like in the following diagram.

Hibernate @Subselect annotation

Knowing how to query the PostgreSQL database functions, our DatabaseFunction can be mapped like this:

@Entity
@Immutable
@Subselect(
    "SELECT " +
    "    functions.routine_name as name, " +
    "    string_agg(functions.data_type, ',') as params " +
    "FROM (" +
    "    SELECT " +
    "        routines.routine_name, " +
    "        parameters.data_type, " +
    "        parameters.ordinal_position " +
    "    FROM " +
    "        information_schema.routines " +
    "    LEFT JOIN " +
    "        information_schema.parameters " +
    "    ON " +
    "        routines.specific_name = parameters.specific_name " +
    "    WHERE " +
    "        routines.specific_schema='public' " +
    "    ORDER BY " +
    "        routines.routine_name, " +
    "        parameters.ordinal_position " +
    ") AS functions " +
    "GROUP BY functions.routine_name"
)
public class DatabaseFunction {

    @Id
    private String name;

    private String params;

    public String getName() {
        return name;
    }

    public String[] getParams() {
        return params.split(",");
    }
}

The @Subselect Hibernate-specific annotation allows you to map a read-only entity directly to the ResultSet of a given SQL query.

Notice that the entity is mapped with the @Immutable annotation since the query is just a read-only projection.

Database view

You can also encapsulate the query in a database view, like this:

CREATE OR REPLACE VIEW database_functions AS
    SELECT 
        functions.routine_name as name,
        string_agg(functions.data_type, ',') as params
    FROM (
        SELECT 
            routines.routine_name, 
            parameters.data_type, 
            parameters.ordinal_position
        FROM 
            information_schema.routines
        LEFT JOIN 
            information_schema.parameters 
        ON 
            routines.specific_name = parameters.specific_name
        WHERE 
            routines.specific_schema='public'
        ORDER BY routines.routine_name, parameters.ordinal_position
    ) AS functions
    GROUP BY functions.routine_name;

Mapping a JPA entity to a database view is even simpler and can be done using plain JPA mappings:

@Entity
@Immutable
@Table(name = "database_functions")
public class DatabaseFunction {

    @Id
    private String name;

    private String params;

    public String getName() {
        return name;
    }

    public String[] getParams() {
        return params.split(",");
    }
}

Testing time

We can query the DatabaseFunction entity using JPQL, as illustrated by the following example:

List<DatabaseFunction> databaseFunctions = 
entityManager.createQuery(
    "select df " +
    "from DatabaseFunction df", DatabaseFunction.class)
.getResultList();

DatabaseFunction countComments = databaseFunctions.get(0);

assertEquals(
    "count_comments", 
    countComments.getName()
);
assertEquals(
    2, 
    countComments.getParams().length
);
assertEquals(
    "bigint", 
    countComments.getParams()[0]
);

DatabaseFunction postComments = databaseFunctions.get(1);

assertEquals(
    "post_comments", 
    postComments.getName()
);
assertEquals(
    1,  
    postComments.getParams().length
);
assertEquals(
    "bigint", 
    postComments.getParams()[0]
);

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That’s it!

Conclusion

Mapping an entity to an SQL result set is actually very easy with JPA and Hibernate.

You can either use the Hibernate-specific @Subselect annotation, in case you don’t want to map the query to a database view. Or, if you map the query as a view, you can simply use that instead of an actual database table.

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