diff --git a/addons/sale_timesheet/models/project.py b/addons/sale_timesheet/models/project.py
index 2a28a279393..d8b4cca03a2 100644
--- a/addons/sale_timesheet/models/project.py
+++ b/addons/sale_timesheet/models/project.py
@@ -56,6 +56,13 @@ class Project(models.Model):
 class ProjectTask(models.Model):
     _inherit = "project.task"
 
+    def _get_default_partner(self):
+        partner = super(ProjectTask, self)._get_default_partner()
+        if 'default_project_id' in self.env.context:  # partner from SO line is prior on one from project
+            project = self.env['project.project'].browse(self.env.context['default_project_id'])
+            partner = project.sale_line_id.order_partner_id
+        return partner
+
     @api.model
     def _default_sale_line_id(self):
         sale_line_id = False
@@ -67,6 +74,15 @@ class ProjectTask(models.Model):
 
     sale_line_id = fields.Many2one('sale.order.line', 'Sales Order Item', default=_default_sale_line_id, domain="[('is_service', '=', True), ('order_partner_id', '=', partner_id), ('is_expense', '=', False)]")
 
+    @api.onchange('project_id')
+    def _onchange_project(self):
+        result = super(ProjectTask, self)._onchange_project()
+        if self.project_id:
+            self.sale_line_id = self.project_id.sale_line_id
+            if not self.partner_id:
+                self.partner_id = self.sale_line_id.order_partner_id
+        return result
+
     @api.multi
     @api.constrains('sale_line_id')
     def _check_sale_line_type(self):
diff --git a/addons/sale_timesheet/tests/test_sale_service.py b/addons/sale_timesheet/tests/test_sale_service.py
index 98378df0041..49cc195da90 100644
--- a/addons/sale_timesheet/tests/test_sale_service.py
+++ b/addons/sale_timesheet/tests/test_sale_service.py
@@ -332,3 +332,44 @@ class TestSaleService(TestCommonSaleTimesheetNoChart):
         self.assertEqual(so_line1.project_id.sale_line_id, so_line1, "SO line of project with template A should be the one that create it.")
         self.assertEqual(so_line2.project_id.sale_line_id, so_line2, "SO line of project should be the one that create it.")
         self.assertEqual(so_line5.project_id.sale_line_id, so_line5, "SO line of project with template B should be the one that create it.")
+
+    def test_billable_task_and_subtask(self):
+        """ Test if subtasks and tasks are billed on the correct SO line """
+        # create SO line and confirm it
+        so_line_deliver_new_task_project = self.env['sale.order.line'].create({
+            'name': self.product_delivery_timesheet3.name,
+            'product_id': self.product_delivery_timesheet3.id,
+            'product_uom_qty': 10,
+            'product_uom': self.product_delivery_timesheet3.uom_id.id,
+            'price_unit': self.product_delivery_timesheet3.list_price,
+            'order_id': self.sale_order.id,
+        })
+        so_line_deliver_new_task_project.product_id_change()
+        self.sale_order.action_confirm()
+
+        project = so_line_deliver_new_task_project.project_id
+        task = so_line_deliver_new_task_project.task_id
+
+        self.assertEqual(project.sale_line_id, so_line_deliver_new_task_project, "The created project should be linked to the so line")
+        self.assertEqual(task.sale_line_id, so_line_deliver_new_task_project, "The created task should be linked to the so line")
+
+        # create a new task and subtask
+        subtask = self.env['project.task'].create({
+            'parent_id': task.id,
+            'project_id': project.id,
+            'name': '%s: substask1' % (task.name,),
+            'sale_line_id': False  # forcing, but should have not effect
+        })
+        task2 = self.env['project.task'].create({
+            'project_id': project.id,
+            'name': '%s: substask1' % (task.name,)
+        })
+
+        self.assertEqual(subtask.sale_line_id, task.sale_line_id, "A child task should have the same SO line than its mother")
+        self.assertEqual(task2.sale_line_id, project.sale_line_id, "A new task in a billable project should have the same SO line than its project")
+        self.assertEqual(task2.partner_id, so_line_deliver_new_task_project.order_partner_id, "A new task in a billable project should have the same SO line than its project")
+
+        # moving subtask in another project
+        subtask.write({'project_id': self.project_global.id})
+
+        self.assertEqual(subtask.sale_line_id, task.sale_line_id, "A child task should always have the same SO line than its mother, even when changing project")