JPA and stored procedures

In a majority number of cases the Java-Persistence API (JPA) to create/load/update entities cover all your needs. But sometimes you need to do something special inside the DB with help of stored-procedures. The good thing about JPA is that you still can use it to call the stored procedures and get the results back. Here is two cases of using it

1. execute the stored-procedure and get the results back as Entity
2. execute stored procedure and get the result back as non-entity (for example set of name-value pairs)

For the 1-st case you need to define the stored procedure in DB (syntax depends on DB, I’ll use the Postgre-SQL syntax)
In the code example below I’ll show how to create a new entity using the call to the stored-procedure. It solve the real problem of how to generate unique-record in DB. If we’ll try to use the standard JPA API we may end up with the code like that

public UniqueCode generateUniqueCode(int length, String prefix) {
do {
    String code = generateRandomString(length, prefix);
    if  ( notExistsInDB() // call to entityManager to check if such code exists) {
        UniqueCode code = new UniqueCode(code);
        entityManager.persist(code);
        return code;
    }
} while(true);
//no way to get to that point
return null;
}

So, at least 2 calls to DB (+1 for each loop cycle). Quite non-optimal, so why not do that with a single call to DB ?
Just in case – no need to inspect SQL code example below

DROP TYPE IF EXISTS unique_code_result CASCADE ;

-- we define the result-type of returned data (postgre need a type to be returned, it can't return RECORD)
CREATE TYPE unique_code_result AS (pk bigint, code character varying);

-- call generate_unique_code_on_table to generate unique value, insert it in the table
-- AND return the result of INSERT command as "unique_code_result" type
CREATE OR REPLACE FUNCTION generate_unique_code(length integer, prefix varchar)
RETURNS unique_code_result AS
$BODY$
DECLARE
 new_record unique_code_result;
BEGIN

  INSERT INTO unique_code (pk, code) VALUES
    (nextVal('unique_code_s'), generate_unique_code_on_table('unique_code', 'code', length, prefix))
  RETURNING pk, code INTO new_record;

RETURN new_record;

END;
$BODY$
LANGUAGE plpgsql VOLATILE
COST 100;

-- generate_unique_code_on_table generates the UNIQUE column-value string for the "unique_column" on the table "table_name".
-- to implement that it generate the random string in the cycle and check if it is already exists by SQL request
-- If it doesn't exists - the value is returned, otherwise it generate next random value and check it
CREATE OR REPLACE FUNCTION generate_unique_code_on_table(table_name varchar, unique_column varchar, code_length integer, prefix varchar)
  RETURNS varchar AS
$BODY$
DECLARE
    select_request varchar;
    result VARCHAR;
    unique_code_key bigint;
BEGIN
    select_request = 'SELECT pk FROM ' || table_name || ' WHERE ' || unique_column || ' = $1';
    LOOP
        result := random_string(code_length);
        IF (prefix IS NOT NULL) THEN
	   result := prefix || result;
        END IF;

        EXECUTE select_request INTO unique_code_key USING result;
        IF NOT FOUND THEN
            RETURN result;
        END IF;
    END LOOP;
END;
$BODY$
  LANGUAGE plpgsql VOLATILE
  COST 100;

Sorry for that long DB code, I want to show the real-life problem solution, not just plain INSERT.
Now, let’s call it from the JPA. In our example we generate and insert new record into the “unique_code” table. Let’s define that entity and required JPA annotations to call the stored procedure.

import javax.persistence.*;

@Entity
//result-type mapping used by native-query call
@SqlResultSetMapping(name = "UniqueCode.implicit", entities = {
        @EntityResult(entityClass = UniqueCode.class)
    }
)
@NamedNativeQueries({
        @NamedNativeQuery(name = "UniqueCode.generateNew",
                query = "select * from generate_unique_code (:length, :prefix)",
                resultSetMapping = "UniqueCode.implicit")
    }
)
@Table(name = "unique_code")
public class UniqueCode {

    @Id
    private Long pk;

    @Column(unique = true)
    private String code;
}

Now let’s call the Native-Query to generate the UniqueCode

        Query query = entityManager.createNamedQuery("UniqueCode.generateNew")
                .setParameter("length", length)
                .setParameter("prefix", prefix != null ? prefix : "");

        return (UniqueCode) query.getSingleResult();

That’s it! This call will return us new Entity which already exists in DB and can be used by other entities. Please note that we have to pass the empty-string (not null) as “prefix”, since DB will not be able to find corresponding db-procedure by signature if we’ll pass NULL.

It’s too much code already here, so I’ll show the 2-nd case “using native-query to return non-entities” in the next post.

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