Add unique constraint to story_tags table

This is a mapping table, we should not have duplicate entries

Change-Id: I5b4548bdd39b9855919653b701efc5f357233a9f

boartty/alembic/versions/acca67ec622b_update_story_tag_table.py

@@ -0,0 +1,23 @@
+"""update story_tag table
+
+Revision ID: acca67ec622b
+Revises: 183755ac91df
+Create Date: 2017-11-27 10:49:04.902131
+
+"""
+
+# revision identifiers, used by Alembic.
+revision = 'acca67ec622b'
+down_revision = '183755ac91df'
+
+from alembic import op
+import sqlalchemy as sa
+
+
+def upgrade():
+    op.create_index(op.f('story_tag_unique'), 'story_tag',
+                    ['story_key', 'tag_key'], unique=True)
+
+
+def downgrade():
+    pass

boartty/db.py

@@ -146,6 +146,7 @@ story_tag_table = Table(
     Column('key', Integer, primary_key=True),
     Column('story_key', Integer, ForeignKey("story.key"), index=True),
     Column('tag_key', Integer, ForeignKey("tag.key"), index=True),
+    UniqueConstraint('story_key', 'tag_key', name='story_tag_unique'),
 )
 task_table = Table(
     'task', metadata,