eager-loading queries with GORM/Hibernate
Question
My Grails app has the following domain objects
class ProductType {
String name
static hasMany = [attributes: Attribute]
}
class Attribute {
String name
static belongsTo = [productType: ProductType]
}
My DB has 7 ProductType
s and each of those has 3 Attribute
s. If I execute the query:
def results = ProductType.withCriteria {
fetchMode("attributes", org.hibernate.FetchMode.EAGER)
}
I expect 7 instances of ProductType
to be returned, but in fact I get 21 (7 x 3). I understand that if I were to execute an equivalent SQL query to the above, the result set would have 21 rows
prod1 | attr1
prod1 | attr2
prod1 | attr3
..... | .....
..... | .....
prod7 | attr1
prod7 | attr2
prod7 | attr3
-------------
Total 21
But I thought that when I retrieve these results via Hibernate/GORM I should get something more like:
prod1 | attr1, attr2, attr3
..... | ...................
..... | ...................
prod7 | attr1, attr2, attr3
---------------------------
Total 7
Incidentally, if I remove the eager-loading from the query above, I get 7 ProductType
s as expected. What am I missing?
Solution
you should read this faq: Hibernate does not return distinct results for a query with outer join fetching enabled for a collection (even if I use the distinct keyword)?
When you specify eager loading, the resultset contains, as you noticed, 7*3 rows but in fact you only have 7 productTypes objects in memory (& 2 extra references for each).
To do what you want, you can add (be aware that the underlying sql query did not change):
SetResultTransformer(new DistinctRootEntityResultTransformer())
def results = ProductType.withCriteria {
fetchMode("attributes", org.hibernate.FetchMode.EAGER)
SetResultTransformer(new DistinctRootEntityResultTransformer())
}