old_contents: from sqlalchemy import Column, ForeignKey, Integer, String, Float
  
  from htsohm.db import Base
  
  class VoidFraction(Base):
      __tablename__ = "void_fractions"
  
      id = Column(Integer, primary_key=True)
  
      # relationship with `materials`
      material_id = Column(Integer, ForeignKey("materials.id"))
  
      # simulation input
      adsorbate      = Column(String(16))
      temperature    = Column(Float)
  
      # simulation output
      void_fraction = Column(Float)
  
new_contents: from sqlalchemy import Column, ForeignKey, Integer, String, Float
  
  from htsohm.db import Base
  
  class VoidFraction(Base):
      __tablename__ = "void_fractions"
  
      id = Column(Integer, primary_key=True)
  
      # relationship with `materials`
      material_id = Column(Integer, ForeignKey("materials.id"))
  
      # simulation input
      adsorbate      = Column(String(16))
      temperature    = Column(Float)
  
      # simulation output
      void_fraction = Column(Float)
  
      def __repr__(self):
          return "(%s: %s-%s p: %s)" % (str(self.id), self.material_id,
                      self.adsorbate, self.temperature, self.void_fraction)
  
subject: add repr for void fraction