old_contents: """alter user
  
  Revision ID: 0974132183ad
  Revises: 
  Create Date: 2017-07-21 18:40:24.918345
  
  """
  from alembic import op
  import sqlalchemy as sa
  
  
  # revision identifiers, used by Alembic.
  revision = '0974132183ad'
  down_revision = None
  branch_labels = None
  depends_on = None
  
  
  def upgrade():
      op.add_column('users', sa.Column('openid', sa.Unicode(128), unique=True))
      op.add_column('users', sa.Column('credential', sa.Unicode(128), unique=True))
      op.add_column('users', sa.Column('cert_expires', sa.TIMESTAMP(timezone=False),default=sa.sql.func.now(),server_default=sa.func.now()))
  
  def downgrade():
      pass
  
  
new_contents: """alter user
  
  Revision ID: 0974132183ad
  Revises: 
  Create Date: 2017-07-21 18:40:24.918345
  
  """
  from alembic import op
  import sqlalchemy as sa
  
  
  # revision identifiers, used by Alembic.
  revision = '0974132183ad'
  down_revision = None
  branch_labels = None
  depends_on = '2bb1ba973f0b'
  
  
  def upgrade():
      op.add_column('users', sa.Column('openid', sa.Unicode(128), unique=True))
      op.add_column('users', sa.Column('credential', sa.Unicode(128), unique=True))
      op.add_column('users', sa.Column('cert_expires', sa.TIMESTAMP(timezone=False),default=sa.sql.func.now(),server_default=sa.func.now()))
  
  def downgrade():
      pass
  
  
subject: add dependence in migration files