Postgres JSON equivalent to HSTORE subtract operator - json

Question

Postgres' hstore extension has a neat subtraction operator:
hstore - text[]
hstore - hstore
In the first case, it removes the key/value pairs where the keys are found in the array of strings: in the second case it removes all matching key/value pairs from the first hstore that appear in the second hstore.
It seems this operator does not exist for the new jsonb data type. Is there a simple way to perform these tasks?

Answer 1

The key is the json_each() function, and the ability in PostgreSQL to manually build up a json value.
Here is a function which can handle json - text[]:
CREATE OR REPLACE FUNCTION "json_object_delete_keys"(
"json" json,
VARIADIC "keys_to_delete" TEXT[]
)
RETURNS json
LANGUAGE sql
IMMUTABLE
STRICT
AS $function$
SELECT COALESCE(
(SELECT ('{' || string_agg(to_json("key") || ':' || "value", ',') || '}')
FROM json_each("json")
WHERE "key" <> ALL ("keys_to_delete")),
'{}'
)::json
$function$;
To handle the json - json case, you simple need to change the WHERE clause:
WHERE "json"->>"key" <> ("remove"->>"key")),

Answer 2

Accepted answer is great, but would be improved for the json - json case by checking for null as well:
WHERE NOT null_as_value_cmp((this_j->>"key"), (that_j->>"key"))
Without the NULL check you get {} instead of {"a":1}:
# select json_subtract('{"a":1, "b":2}'::json, '{"b":2}'::json);
json_subtract
---------------
{}
(1 row)
null_as_value_cmp is something like this and gets around JsNull being represented as the database NULL
CREATE OR REPLACE FUNCTION null_as_value_cmp(
a text,
b text
)
RETURNS boolean
LANGUAGE sql
IMMUTABLE
CALLED ON NULL INPUT
AS $function$
SELECT CASE
WHEN a IS NULL AND b IS NULL THEN
TRUE
WHEN (a IS NULL AND b IS NOT NULL) THEN
FALSE
WHEN (a IS NOT NULL AND b IS NULL) THEN
FALSE
WHEN a = b THEN
TRUE
ELSE
FALSE
END;
$function$;
[I don't have enough reputation to comment; not sure on the SO protocol here.]