Propel2与表连接，然后按聚合函数分组

Question

我有两个表：food（id，name），review（user_id，food_id，rating）。 现在我想将食物表与评论表连接起来，并在虚拟食物表中添加名称为avg_rating的虚拟列，该虚拟列显然将保留基于评论的食物的平均评分值。 所以我的想法是做这样的事情：

$food = FoodQuery::create()
        ->filterById(15) // constant for testing purposes only
        ->leftJoinWithReview()
        ->withColumn("AVG(review.rating)", "avg_rating")
        ->groupBy("review.rating")
        ->find()

现在在调试器中，我看到以下内容：

result = {Propel\Runtime\Collection\ObjectCollection} [7]
 index = {array} [1]
 indexSplHash = {array} [1]
 model = "Food"
 fullyQualifiedModel = \Food
 formatter = {Propel\Runtime\Formatter\ObjectFormatter} [10]
 data = {array} [1]
  0 = {\Food} [34]
   new = false
   deleted = false
   modifiedColumns = {array} [0]
   virtualColumns = {array} [1]
    avg_rating = "5.0000"
   id = 15
   name = "Salát Caesar"
   collReviews = {Propel\Runtime\Collection\ObjectCollection} [7]
    index = {array} [2]
    indexSplHash = {array} [2]
    model = "Review"
    fullyQualifiedModel = "\Review"
    formatter = null
    data = {array} [2]
     0 = {\Review} [14]
      new = false
      deleted = false
      modifiedColumns = {array} [0]
      virtualColumns = {array} [0]
      user_id = 1
      food_id = 15
      rating = 3
      aFood = {\Food} [34]
      aUser = null
      alreadyInSave = false
      reviewThumbsUpsScheduledForDeletion = null
     1 = {\Review} [14]
      new = false
      deleted = false
      modifiedColumns = {array} [0]
      virtualColumns = {array} [0]
      user_id = 3
      food_id = 15
      rating = 5
      aFood = {\Food} [34]
      aUser = null
      alreadyInSave = false
      reviewThumbsUpsScheduledForDeletion = null
    *Propel\Runtime\Collection\Collection*pluralizer = null
   collReviewsPartial = false
   alreadyInSave = false
   reviewsScheduledForDeletion = null
 *Propel\Runtime\Collection\Collecti4

问题是平均评分不正确。 在virtualColumns中，您可以看到avg_rating字段，其值为“ 5.0000”。 但是，当您看上去较低时，您实际上可以看到该食物有2条评论的评分分别为3和5，因此平均值应为“ 4.0000”。

问题出在哪儿？ 为什么这不能正常工作？

Answer 1

您当前正在对审阅表进行左联接。 左连接将导致返回几行（每条评论一个）。 对于返回的每一行，只有一个评论，因此“平均”将与该行的评论分数相同。 按平均值分组不会做很多事情，只会将分数完全相同的评论分组，这不是您要查找的内容。

您应该先按食物ID分组。 然后，您将可以对每个食品获得一行，并获得平均评分。

$food = FoodQuery::create()
    ->filterById(15) // constant for testing purposes only
    ->groupById()
    ->leftJoinWithReview()
    ->withColumn("AVG(review.rating)", "avg_rating")
    ->find();