Sort by two fields added together

[Arbee_N_A]

I’m currently using collection.find().sort(Sorts.descending("x", "y")).skip(page * limit).limit(limit), but how can I sort by whatever “x” and “y” is added together? (Java)

[steevej (Steeve Juneau)]

If I understand correctly your documents have two fields such as:

{ _id : 0 ,
  foo : 10 ,
  bar : 20 }
{ _id : 1 ,
  foo : 5 ,
  bar : 35 }

and you want to sort based on the result of adding foo to bar.

I do not think you can do that with find() and sort(). You need aggregation:

set_sum = { "$set" : { "_sum" : { "$sum" : [ "$foo" , "$bar" ] } } }
sort_sum = { "$sort" : { "_sum" : -1 } }
skip = { "$skip" : page * limit }
limit = { "$limit" : limit }
pipeline = [ set_sum , sort_sum , skip , limit ]

I do not use the Builders so I cannot give you the exact syntax but Compass can take the pipeline and export it to Java code that uses the builders.

Since you are sorting on a computed value a collection scan will be performed. You might want to store the sum permanently and have an index if you have performance issues.

[Arbee_N_A]

Thanks you, that worked… Also thanks for mentioning that Compass can do that… Wouldn’t have been able to figure it out without that ._: