How to use MongoDB aggregation for general purpose set operations (union, intersection, difference)

Question

I have come across some special purpose implementation of set operations, but nothing for the general case. What is the general case for performing set operations (specific

Answer 1

Version 2.6+ Only:

As of version 2.6 of MongoDB, this has become much much easier. You can now do the following to solve this problem:

Union

db.colors.aggregate([
    {'$project': {  
                    union:{$setUnion:["$left","$right"]}
                 }
    }
]);

Intersection

db.colors.aggregate([
    {'$project': {  
                  int:{$setIntersection:["$left","$right"]}
                 }
    }
]);

Relative Complement

db.colors.aggregate([
    {'$project': {  
                    diff:{$setDifference:["$left","$right"]}
                 }
    }
]);

Symmetric Difference

db.colors.aggregate([
    {'$project': {  
                    diff:{$setUnion:[{$setDifference:["$left","$right"]}, {$setDifference:["$right","$left"]}]}
                 }
    }
]);

Note: There is a ticket requesting symmetric difference be added as a core feature rather than having to do the union of two set differences.