docs: def upgrade():
      """Upgrade database."""
      op.create_table(
          'collection',
          sa.Column('id', sa.Integer(), nullable=False),
          sa.Column('name', sa.String(length=255), nullable=False),
          sa.Column('dbquery', sa.Text(), nullable=True),
          sa.Column('rgt', sa.Integer(), nullable=False),
          sa.Column('lft', sa.Integer(), nullable=False),
          sa.Column('level', sa.Integer(), nullable=False),
          sa.Column('parent_id', sa.Integer(), nullable=True),
          sa.Column('tree_id', sa.Integer(), nullable=True),
          sa.ForeignKeyConstraint(
              ['parent_id'], ['collection.id'], ondelete='CASCADE'
          ),
          sa.PrimaryKeyConstraint('id')
      )
      op.create_index(
          'collection_level_idx', 'collection', ['level'], unique=False
      )
      op.create_index('collection_lft_idx', 'collection', ['lft'], unique=False)
      op.create_index('collection_rgt_idx', 'collection', ['rgt'], unique=False)
      op.create_index(
          op.f('ix_collection_name'), 'collection', ['name'], unique=True
      )