code: func (c *expressionCompiler) expressionRefersToJoinedData(e criteria.Expression) (*TableJoin, bool) {
  	switch t := e.(type) {
  	case *criteria.FieldExpression:
  		for _, j := range c.joins {
  			if j.HandlesFieldName(t.FieldName) {
  				j.Active = true
  				return j, true
  			}
  		}
  	}
  	return nil, false
  }
docs: // expressionRefersToJoinedData returns true if the given field expression is a
  // field expression and  refers to joined data; otherwise false is returned.