code: func (q *Query) BelongsToThrough(bt, thru interface{}) *Query {
  	q.belongsToThroughClauses = append(q.belongsToThroughClauses, belongsToThroughClause{
  		BelongsTo: &Model{Value: bt},
  		Through:   &Model{Value: thru},
  	})
  	return q
  }
docs: // BelongsToThrough adds a "where" clause that connects the "bt" model
  // through the associated "thru" model.