Using Postgres FK from jsonb with Hasura? - postgresql

Question

We have foreign keys within a json blob in postgres. We join with these like so:
SELECT f.id, b.id FROM foo AS f
LEFT JOIN bar AS b ON f.data -> 'baz' ->> 'barId' = text(b.id)
I'm now trying out Hasura to do som graphql queries and I need these as object relationships. In the UI I can only try to manually add relationships with normal columns, not nested json data:
Is it at all possible to get a graphql relationship this way?

Answer 1

I got the answer in comments, thanks #iamnat. I'll just evolve here with my example for clarity since I still struggled a bit:
Super simple schema and data as such:
CREATE EXTENSION IF NOT EXISTS "uuid-ossp";
CREATE TABLE foo
(
id UUID PRIMARY KEY DEFAULT uuid_generate_v4(),
name text,
data jsonb NOT NULL
);
CREATE TABLE bar
(
id UUID PRIMARY KEY DEFAULT uuid_generate_v4(),
name text,
);
WITH bars AS (
INSERT INTO bar (name) VALUES ('bar') RETURNING id
)
INSERT INTO foo (name, data) VALUES ('foo', jsonb_build_object('barId', (SELECT id FROM bars)));
I then can create a function for the relationship:
CREATE FUNCTION foo_bar(foo_row foo)
RETURNS SETOF bar AS $$
SELECT *
FROM bar
WHERE text(id) = foo_row.data ->> 'barId'
$$ LANGUAGE sql STABLE;
This I can then use in Hasura as a computed field under "Data" -> foo -> Modify -> Computed fields -> "Add a new computed field". Just give it a name and reference the function in a dropdown:
I can then query:
query MyQuery {
foo {
name
foo_bar {
name
}
}
}
with expected result:
{
"data": {
"foo": [
{
"name": "foo",
"foo_bar": [
{
"name": "bar"
}
]
}
]
}
}