Add "root_execution" mapped property to WorkflowExecution model

* We need to avoid using direct DB queries w/o using a mapped
  model where possible for performance sake. Using a mapped entity
  property is always more efficient because it's cached in an
  SQLAlchemy session.

Change-Id: I2d7652ea0cff8f2db7259d285ac98c582bf15b62

mistral/db/v2/sqlalchemy/models.py

@@ -353,6 +353,13 @@ WorkflowExecution.root_execution_id = sa.Column(
     nullable=True
 )
 
+WorkflowExecution.root_execution = relationship(
+    WorkflowExecution,
+    remote_side=WorkflowExecution.id,
+    lazy='select'
+)
+
+
 # Many-to-one for 'TaskExecution' and 'WorkflowExecution'.
 
 TaskExecution.workflow_execution_id = sa.Column(

mistral/tests/unit/engine/test_subworkflows.py

@@ -453,9 +453,11 @@ class SubworkflowsTest(base.EngineTestCase):
             wf2_ex = db_api.get_workflow_execution(wf2_ex.id)
             wf3_ex = db_api.get_workflow_execution(wf3_ex.id)
 
-        self.assertIsNone(wf1_ex.root_execution_id, None)
-        self.assertEqual(wf2_ex.root_execution_id, wf1_ex.id)
-        self.assertEqual(wf3_ex.root_execution_id, wf1_ex.id)
+            self.assertIsNone(wf1_ex.root_execution_id, None)
+            self.assertEqual(wf2_ex.root_execution_id, wf1_ex.id)
+            self.assertEqual(wf2_ex.root_execution, wf1_ex)
+            self.assertEqual(wf3_ex.root_execution_id, wf1_ex.id)
+            self.assertEqual(wf3_ex.root_execution, wf1_ex)
 
     def test_cascade_delete(self):
         wf_text = """

mistral/workflow/data_flow.py

@@ -17,7 +17,6 @@ from oslo_config import cfg
 from oslo_log import log as logging
 
 from mistral import context as auth_ctx
-from mistral.db.v2 import api as db_api
 from mistral.db.v2.sqlalchemy import models
 from mistral import exceptions as exc
 from mistral import expressions as expr
@@ -333,9 +332,7 @@ def get_workflow_environment_dict(wf_ex):
         return {}
 
     if wf_ex.root_execution_id:
-        return get_workflow_environment_dict(
-            db_api.get_workflow_execution(wf_ex.root_execution_id)
-        )
+        return get_workflow_environment_dict(wf_ex.root_execution)
 
     env_dict = wf_ex.params['env'] if 'env' in wf_ex.params else {}