[{"name":"T1.1","a":1,"products":[{"name":"T1.1","code":123},{"name":"foo","code":321}]},{"name":"T1.2","a":4,"products":[{"name":"T1.1","code":123},{"name":"foo","code":321}]}]

db.collection.aggregate([{$match:{name:{$regex:"^T1.*"}}},{$group:{"_id":null,"avg":{"$avg":"$a"}}}])