code: def upgrade():
      op.add_column(
          "dashboards",
          sa.Column("slug", sa.String(length=255), nullable=True),
      )
      try:
          op.create_unique_constraint(
              "idx_unique_slug", "dashboards", ["slug"]
          )
      except:
          pass
  
imports: import sqlalchemy
  from alembic import op
label_desc: Use image node_id 1 for calling a global function with example usage: upgrade() without return types