title: Sequelize Model where value is set using a PostgreSQL/PostGIS function
body: <p>Is it possible to set up a Sequelize model in such a way that an instance is written to the database using a PostgreSQL (or PostGIS, in my case) function?</p>
  
  <p>My database leverages PostGIS, and I would like for the value for "the_geom" to be written as a function of the latitude and longitude:</p>
  
  <pre><code>var Site = sequelize.define('site', {
    the_geom: 'geometry(Point,4326)',
    latitude: DataTypes.DECIMAL,
    longitude: DataTypes.DECIMAL
  });
  </code></pre>
  
  <p>The value for the_geom would typically be something like this:</p>
  
  <pre><code>ST_SetSRID(ST_MakePoint(-76.622,39.277),4326)
  </code></pre>