code: def upgrade():
      
      op.create_table(
          'pidrelations_pidrelation',
          sa.Column('created', sa.DateTime(), nullable=False),
          sa.Column('updated', sa.DateTime(), nullable=False),
          sa.Column('parent_id', sa.Integer(), nullable=False),
          sa.Column('child_id', sa.Integer(), nullable=False),
          sa.Column('relation_type', sa.SmallInteger(), nullable=False),
          sa.Column('index', sa.Integer(), nullable=True),
          sa.ForeignKeyConstraint(
              ['child_id'], ['pidstore_pid.id'],
              name=op.f('fk_pidrelations_pidrelation_child_id_pidstore_pid'),
              onupdate='CASCADE', ondelete='CASCADE'
          ),
          sa.ForeignKeyConstraint(
              ['parent_id'], ['pidstore_pid.id'],
              name=op.f('fk_pidrelations_pidrelation_parent_id_pidstore_pid'),
              onupdate='CASCADE', ondelete='CASCADE'),
          sa.PrimaryKeyConstraint(
              'parent_id', 'child_id', name=op.f('pk_pidrelations_pidrelation')
          )
      )
docs: Upgrade database.