Switch QueueItem state_id to be unique after a backfill

data/migrations/versions/d42c175b439a_backfill_state_id_and_make_it_unique.py

@@ -0,0 +1,33 @@
+"""Backfill state_id and make it unique
+
+Revision ID: d42c175b439a
+Revises: 3e8cc74a1e7b
+Create Date: 2017-01-18 15:11:01.635632
+
+"""
+
+# revision identifiers, used by Alembic.
+revision = 'd42c175b439a'
+down_revision = '3e8cc74a1e7b'
+
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects import mysql
+
+def upgrade(tables):
+    # Backfill the queueitem table's state_id field with unique values for all entries which are
+    # empty.
+    conn = op.get_bind()
+    conn.execute("update queueitem set state_id = id where state_id = ''")
+
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.drop_index('queueitem_state_id', table_name='queueitem')
+    op.create_index('queueitem_state_id', 'queueitem', ['state_id'], unique=True)
+    # ### end Alembic commands ###
+
+
+def downgrade(tables):
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.drop_index('queueitem_state_id', table_name='queueitem')
+    op.create_index('queueitem_state_id', 'queueitem', ['state_id'], unique=False)
+    # ### end Alembic commands ###