Aggregate function on "large value type"

Question

We are constrained to use a model that stores records in a main table and various attributes of the record in a second table. Something like this: main record table CAR, attribute table CAR_DETAILS with CAR_DETAILS have key value pairs like "color"
"blue" and "doors" "4".
For reports we need to flatten the one to many nature of this to get CAR and color and doors by using an aggregate function like: 
car_id, max(car_detail.value) filtered on car_detail.key = "color", max(car_detail.value) filtered on car_detail.key = "doors". 
This works on other tables but the car_detail table (shall we say) appears to store its values as blobs. In any case when we attempt to aggregate we get "The query uses an aggregate function on a large value type expression. Large value type expressions
can not be aggregated."
Since we can't change the model, we would need to use another function to change this to a smaller string (or date, these are actually mostly date), but none of the very limited set of functions available seems to work to make this aggregation possible (and
there is no "first" or anything else similar). 
The list of functions available in Report Model Queries can be found at https://msdn.microsoft.com/en-us/library/ee210538.aspx
Appreciate any ideas on how to solve this
 

Answer 1

I found a work around : create a second dataset reversing the direction:
car_detail.key, car_detail.value, car_id
with this filtered by car_detail.key = "color" (or doors etc)
Then use a lookup from the first dataset to the second to display the data:
=Lookup(Fields!car_Id.Value,Fields!Icar_Id.Value,Fields!car_detail.Value, "DataSet2")
where the 1st field is in the main report dataset, the second and third are in the lookup dataset, and the 4th parameter is the name of the lookup dataset. 
Would still really like to have a solution that allows aggregation for ease of use and efficiency.