Tentative fix for #1128

The subquery that builds the final contract tree when querying with
links uses an inner join to get the nested link. Roughly:

```
JOIN (
  // Build the linked contract...
) AS linked
ON edge.source = linked.id
```

These joins are nested in the case of nested links and there is a
toplevel `LATERAL`. In the case of two nested optional links, the
`LATERAL` subquery looks roughly like this:

```
FROM
  cards,
  LATERAL (
    // ...
    JOIN (
      // ...
    ) AS linked
    ON edge1.source = linked.id
    // ...
  )
WHERE edge0.source = cards.id
```

If the optional contracts don't exist, `linked.id` will be null and the
`edge1.to = linked.id` condition will fail. As a consequence, the whole
lateral will be empty and so will the result. The fix is change those
inner joins into left joins.

This is a tentative fix because I have fuzzy memories of making this
exact change and causing unrelated tests to fail.

Fixes #1128
Change-type: patch
Signed-off-by: Carol Schulze <carol@balena.io>

lib/backend/postgres/jsonschema2sql/sql-query.ts

@@ -279,7 +279,7 @@ const pushLinkedLateral = (
 		.pushLeftJoin(cardsTable, lateralJoinFilter, 'linked')
 		.pushGroupBy('orderedEdges', SqlPath.fromArray(['source']));
 	for (const [nestedLateral, nestedLateralAlias] of nestedLaterals) {
-		lateral.pushInnerJoin(
+		lateral.pushLeftJoin(
 			nestedLateral,
 			new LiteralSql(`${nestedLateralAlias}.source = linked.id`),
 			nestedLateralAlias,

test/integration/kernel.spec.ts

@@ -7648,5 +7648,47 @@ describe('Kernel', () => {
 
 			await once(stream, 'closed');
 		});
+
+		it('issue #1128: should be able to query for two nested optional links', async () => {
+			const card = await ctx.kernel.insertCard(
+				ctx.context,
+				ctx.kernel.sessions!.admin,
+				{
+					slug: ctx.generateRandomSlug(),
+					type: 'card@1.0.0',
+				},
+			);
+
+			const results = await ctx.kernel.query(
+				ctx.context,
+				ctx.kernel.sessions!.admin,
+				{
+					properties: {
+						id: {
+							const: card.id,
+						},
+					},
+					anyOf: [
+						{
+							$$links: {
+								'is attached to': {
+									anyOf: [
+										{
+											$$links: {
+												'is attached to': {},
+											},
+										},
+										true,
+									],
+								},
+							},
+						},
+						true,
+					],
+				},
+			);
+
+			expect(results).toEqual([card]);
+		});
 	});
 });