postgres change jsonb[] to jsonb

Question

I use postgres9.4, and there exists relation "Patients" has column "contact" with type jsonb[], how to transfer type jsonb[] to jsonb?

The following is on record.

=>select name, contact from "Patients" where contact is not null;

name  |                                               contact                                               
--------+-----------------------------------------------------------------------------------------------------
"tom" | {"{\"name\": \"tom\", \"phone\": \"111111\", \"address\": \"shanghai\", \"relation\": \"your_relation\"}"}

I have tried as the followings, contact4 is column with type jsonb

alter table "Patients" alter column contact4 type jsonb using contact4::text::jsonb;

ERROR:  invalid input syntax for type json
DETAIL:  Expected ":", but found "}".
CONTEXT:  JSON data, line 1: ...ress\": \"shanghai\", \"relation\": \"your_relation\"}"}

Answer 1

If only the first element of jsonb array is used then the issue is simple:

alter table "Patients" alter column contact type jsonb using contact[1]::jsonb;

else you can use the following function:

create or replace function jsonb_array_to_jsonb(jsonb[])
returns jsonb language sql as $$
    select jsonb_object_agg(key, value)
    from unnest($1), jsonb_each(unnest)
$$;

alter table "Patients" alter column contact type jsonb using jsonb_array_to_jsonb(contact);

Answer 2

As of 9.5 version, to preserve the data and keep the data in json as array this would work much better.

ALTER TABLE "Patients"  ALTER COLUMN "contact" DROP DEFAULT
ALTER TABLE "Patients"  ALTER COLUMN "contact" TYPE jsonb USING to_json(contact)
ALTER TABLE "Patients"  ALTER COLUMN "contact" SET DEFAULT '[]'