Criteria API Average of hasmany Relationship - grails

Question

We have an Shop with ShopArticles and want to add an Rating system.
Our ShopArticle looks like this:
class ShopArticle {
String contUnit = 'STK', orderUnit = 'PCK'
Double value, tax = 0.19
String name, description, keyword
String group1, group2, group3, articleNumber
String producer
Boolean unlocked
static hasMany = [ratings: ShopArticleRating]
}
And the Rating looks like this:
class ShopArticleRating {
String comment
int rating
ShopArticle shopArticle
User user
static belongsTo = ShopArticle
}
Now we want to filter for the average Rating of an atricle, so we made this:
def shopArticleList = ShopArticleRating.createCriteria().listDistinct {
projections {
groupProperty("shopArticle")
}
}
def ids = []
shopArticleList.each { shopArticle ->
def sum = 0
shopArticle.ratings.each {
sum += it.rating
}
if ((sum / shopArticle.ratings.size()) >= filter.rating) {
ids.add(shopArticle.id)
}
}
List<ShopArticle> list = ShopArticle.createCriteria().list {
if (ids.size() > 0) {
'in'("id", ids)
}
}
Is there a better way to filter for the average Rating?
Maybe like this:
List<ShopArticle> list = ShopArticle.createCriteria().list {
createAlias('ratings','r')
projections {
groupProperty('r.rating')
}
gt("r.rating",filter.rating)
}

Answer 1

Were it me, I'd add an averageRating attribute to the ShopArticle class itself.
Do the math to compute the average when a rating is added/deleted/changed for that ShopArticle, and distribute the 'cost' of doing the math across each rating entry/change. Ten people add ratings, you do the math 10 times. A thousand people do queries, you don't do the math 1000 times.
Showing the average rating becomes nothing more than showing another attribute on the screen, filtering is trivial -- no extra work when querying data (and I think it is safe to bet that there will be more queries than ratings added).

Answer 2

Try this:
ShopArticleRating.withCriteria {
projections {
avg("rating")
groupProperty("shopArticle")
}
}
This should give you the article object together with the average of ratings.
I could not test the other way around but it should be like this:
ShopArticle.withCriteria {
projections {
ratings {
avg("rating)
}
property("id")
}
}
which should give you the article id together with the average of the correspondent ratings
another way would be to write your own sql syntax which gives you the absolute freedom to return everything you want. In that case look for "grails HQL". Sometimes I touch the borders when I need to do very complex queries. But in your case you should be good.