orm: fix subquery without where expr (#21598)

vlib/db/sqlite/parent_child_test.v

@@ -0,0 +1,83 @@
+import db.sqlite
+
+struct Parent {
+	id       int     @[primary; sql: serial]
+	name     string
+	children []Child @[fkey: 'parent_id']
+}
+
+struct Child {
+	id        int    @[primary; sql: serial]
+	name      string
+	parent_id int
+	babies    []Baby @[fkey: 'child_id']
+}
+
+struct Baby {
+	id       int    @[primary; sql: serial]
+	name     string
+	child_id int
+}
+
+fn test_main() {
+	db := sqlite.connect(':memory:')!
+
+	sql db {
+		create table Parent
+		create table Child
+		create table Baby
+	} or { panic(err) }
+
+	new_parent := Parent{
+		name: 'first parent'
+		children: [
+			Child{
+				name: 'first child'
+			},
+			Child{
+				name: 'second_child'
+			},
+		]
+	}
+
+	sql db {
+		insert new_parent into Parent
+	} or { panic(err) }
+
+	babies := [
+		Baby{
+			name: 'first baby'
+			child_id: 1
+		},
+		Baby{
+			name: 'second baby'
+			child_id: 1
+		},
+		Baby{
+			name: 'third baby'
+			child_id: 2
+		},
+		Baby{
+			name: 'fourth baby'
+			child_id: 2
+		},
+		Baby{
+			name: 'fifth baby'
+			child_id: 2
+		},
+	]
+
+	for v in babies {
+		sql db {
+			insert v into Baby
+		} or { panic(err) }
+	}
+
+	parent := sql db {
+		select from Parent
+	} or { panic(err) }
+
+	assert parent[0].children[0].id == 1
+	assert parent[0].children[1].id == 2
+	assert parent[0].children.len == 2
+}