orm: add tests for fkey: relationship attribute and retrival of joined rows (#16877)

cmd/tools/vtest-self.v

@@ -142,6 +142,7 @@ const (
 	skip_with_fsanitize_address   = [
 		'vlib/net/websocket/websocket_test.v',
 		'vlib/v/tests/websocket_logger_interface_should_compile_test.v',
+		'vlib/v/tests/orm_sub_array_struct_test.v',
 	]
 	skip_with_fsanitize_undefined = [
 		'do_not_remove',

vlib/v/tests/orm_sub_array_struct_test.v

@@ -7,6 +7,7 @@ struct Parent {
 }
 
 struct Child {
+mut:
 	id        int    [primary; sql: serial]
 	parent_id int
 	name      string
@@ -47,3 +48,68 @@ fn test_orm_array() {
 	assert parent.children[0].name == 'abc'
 	assert parent.children[1].name == 'def'
 }
+
+fn test_orm_relationship() {
+	mut db := sqlite.connect(':memory:') or { panic(err) }
+	sql db {
+		create table Parent
+	}
+
+	mut child := Child{
+		name: 'abc'
+	}
+
+	par := Parent{
+		name: 'test'
+		children: []
+	}
+
+	sql db {
+		insert par into Parent
+	}
+
+	mut parent := sql db {
+		select from Parent where id == 1
+	}
+
+	child.parent_id = parent.id
+	child.name = 'atum'
+
+	sql db {
+		insert child into Child
+	}
+
+	child.name = 'bacon'
+
+	sql db {
+		insert child into Child
+	}
+
+	assert parent.name == par.name
+	assert parent.children.len == 0
+
+	parent = sql db {
+		select from Parent where id == 1
+	}
+
+	assert parent.name == par.name
+	assert parent.children.len == 2
+	assert parent.children[0].name == 'atum'
+	assert parent.children[1].name == 'bacon'
+
+	mut children := sql db {
+		select from Child
+	}
+
+	assert children.len == 2
+
+	sql db {
+		drop table Parent
+	}
+
+	children = sql db {
+		select from Child
+	}
+
+	assert children.len == 0
+}