added uniqueness constraint to human task so it cannot have multiple human tasks for a process instance and task id w/ burnettk (#396)

Co-authored-by: jasquat <jasquat@users.noreply.github.com>

spiffworkflow-backend/migrations/versions/56e17aab85ca_.py

@@ -0,0 +1,32 @@
+"""empty message
+
+Revision ID: 56e17aab85ca
+Revises: 64adf34a98db
+Create Date: 2023-07-14 11:54:20.187703
+
+"""
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision = '56e17aab85ca'
+down_revision = '64adf34a98db'
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    # ### commands auto generated by Alembic - please adjust! ###
+    with op.batch_alter_table('human_task', schema=None) as batch_op:
+        batch_op.create_unique_constraint('process_instance_task_unique', ['process_instance_id', 'task_id'])
+
+    # ### end Alembic commands ###
+
+
+def downgrade():
+    # ### commands auto generated by Alembic - please adjust! ###
+    with op.batch_alter_table('human_task', schema=None) as batch_op:
+        batch_op.drop_constraint('process_instance_task_unique', type_='unique')
+
+    # ### end Alembic commands ###

spiffworkflow-backend/src/spiffworkflow_backend/models/human_task.py

@@ -23,6 +23,14 @@ if TYPE_CHECKING:
 class HumanTaskModel(SpiffworkflowBaseDBModel):
     __tablename__ = "human_task"
 
+    __table_args__ = (
+        db.UniqueConstraint(
+            "process_instance_id",
+            "task_id",
+            name="process_instance_task_unique",
+        ),
+    )
+
     id: int = db.Column(db.Integer, primary_key=True)
     process_instance_id: int = db.Column(
         ForeignKey(ProcessInstanceModel.id), nullable=False, index=True  # type: ignore