[FIXED] Mapping PostgreSQL JSON column to a Hibernate entity property

Issue

I have a table with a column of type JSON in my PostgreSQL DB (9.2). I have a hard time to map this column to a JPA2 Entity field type.

I tried to use String but when I save the entity I get an exception that it can't convert character varying to JSON.

What is the correct value type to use when dealing with a JSON column?

@Entity
public class MyEntity {

    private String jsonPayload; // this maps to a json column

    public MyEntity() {
    }
}

A simple workaround would be to define a text column.

Solution

See PgJDBC bug #265.

PostgreSQL is excessively, annoyingly strict about data type conversions. It won't implicitly cast text even to text-like values such as xml and json.

The strictly correct way to solve this problem is to write a custom Hibernate mapping type that uses the JDBC setObject method. This can be a fair bit of hassle, so you might just want to make PostgreSQL less strict by creating a weaker cast.

As noted by @markdsievers in the comments and this blog post, the original solution in this answer bypasses JSON validation. So it's not really what you want. It's safer to write:

CREATE OR REPLACE FUNCTION json_intext(text) RETURNS json AS $$
SELECT json_in($1::cstring); 
$$ LANGUAGE SQL IMMUTABLE;

CREATE CAST (text AS json) WITH FUNCTION json_intext(text) AS IMPLICIT;

AS IMPLICIT tells PostgreSQL it can convert without being explicitly told to, allowing things like this to work:

regress=# CREATE TABLE jsontext(x json);
CREATE TABLE
regress=# PREPARE test(text) AS INSERT INTO jsontext(x) VALUES ($1);
PREPARE
regress=# EXECUTE test('{}')
INSERT 0 1

Thanks to @markdsievers for pointing out the issue.

Answered By - Craig Ringer