How about this:
db.items.find({x : { $all: [ {$elemMatch: {a: 1, b: 2}}, {$elemMatch: {a: 3, b: 4}}, {$elemMatch: {a: 5, b: 6}} ]}}Check out the Mongo docs.
Also, note the docs warning:
In the current release, queries that use the $all operator must scan all the documents that match the first element in the query array. As a result, even with an index to support the query, the operation may be long running, particularly when the first element in the array is not very selective.