Merging an array of JSON objects to one JSON column in Postgres - postgresql

Question

I have two tables, products and products_ext that can be reduced essentially
to this basic form:
CREATE TABLE "products" (
"product_id" TEXT PRIMARY KEY
);
CREATE TABLE "products_ext" (
"product_id" TEXT NOT NULL,
"key" TEXT NOT NULL,
"value" JSON,
PRIMARY KEY ("product_id", "key"),
FOREIGN KEY ("product_id") REFERENCES "products"("product_id")
ON DELETE CASCADE
ON UPDATE CASCADE
);
Let us assume mock data
INSERT INTO "products" ("product_id") VALUES
('test1'),
('test2'),
('test3');
INSERT INTO "products_ext" (product_id, "key", "value") VALUES
('test1', 'foo', '"Foo"'),
('test1', 'bar', '"Bar"'),
('test2', 'foo', '"Foo"');
I can use a query
SELECT
"P"."product_id",
ARRAY(
SELECT
json_build_object(
"E"."key",
"E"."value"
)
FROM "products_ext" AS "E"
WHERE "E"."product_id" = "P"."product_id"
)
FROM
"products" AS "P";
which yields
product_id | array
------------+-----------------------------------------------
test1 | {"{\"foo\" : \"Foo\"}","{\"bar\" : \"Bar\"}"}
test2 | {"{\"foo\" : \"Foo\"}"}
but I cannot make it to yield a merged JSON. Is there an easy way in Postgres 10
to merge an array of multiple JSONs as one JSON that would yield?
product_id | json
------------+----------------------------------------
test1 | {\"foo\" : \"Foo\", \"bar\" : \"Bar\"}
test2 | {\"foo\" : \"Foo\"}
test3 | {}
Primary key pair "product_id" and "key" already make sure that there are no
key collisions. There may be rows in the products that do not have any data in products_ext and in those cases an empty JSON object should be provided.

Answer 1

demo:db<>fiddle
Use json_object_agg():
SELECT
p.product_id AS product_id,
json_object_agg(e.key, e.value)
FROM
products AS p
JOIN
products_ext AS e ON p.product_id = e.product_id
GROUP BY p.product_id;
Edit for empty product_ext values:
demo:db<>fiddle
SELECT
p.product_id AS product_id,
COALESCE(
json_object_agg(e.key, e.value) FILTER (WHERE e.key IS NOT NULL),
'{}'
)
FROM
products AS p
LEFT JOIN
products_ext AS e ON p.product_id = e.product_id
GROUP BY p.product_id;