I have two tables: food(id, name), review(user_id, food_id, rating). Now I want to join food table with review table and add virtual column to the food table with name avg_rating, that will obviously hold value of average rating of the food based on the reviews. So my idea was to do something like this:
$food = FoodQuery::create()
->filterById(15) // constant for testing purposes only
->leftJoinWithReview()
->withColumn("AVG(review.rating)", "avg_rating")
->groupBy("review.rating")
->find()
Now in debugger I see this:
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
Problem is that average rating is not correct. In virtualColumns you can see avg_rating field with value "5.0000". But when you look little bit lower you can actually see that this food has 2 reviews with rating 3 and 5, so the average should be value "4.0000".
Where is the problem? Why is this not working correctly?
You are currently doing a left join on the review table. Left joining will result in several rows being returned (one per review). For each row returned, there is only one review, so the 'average' will be the same as the review score for that row. Grouping by the average will not do much, it will only group reviews with the exact same score, and that is not what you are looking for.
You should group by your food id first. Then you will be able to get a single row per food item, with an average review score.
$food = FoodQuery::create()
->filterById(15) // constant for testing purposes only
->groupById()
->leftJoinWithReview()
->withColumn("AVG(review.rating)", "avg_rating")
->find();
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