code: function defineRelation(modelName, relationType, targetModelName, options) {
      if (debug) {
          fs.writeSync(debugFD, modelName + '.' + relationType + '(' + targetModelName + ', ' + util.inspect(options, {depth: null}) + ');\n\n');
      }
  
      models[modelName][relationType](models[targetModelName], options); // account.hasMany(contact, {...})
  }
docs: Define 'hasMany' and 'belongsTo' relations.
  @private
  @param {string} modelName - Name of the model
  @param {string} relationType - Type of the relation 'hasMany' or 'belongsTo'
  @param {string} targetModelName - Name of the target model which this relation related to.
  @param {Object} options - Sequelize options of relation.