db.A.find(){ "_id" : ObjectId("5018da521781352fe25bf4d2"), "a" : "1", "b" : "1", "c" : "1", "d" : "1", "e" : "1" }{ "_id" : ObjectId("5018da5a1781352fe25bf4d3"), "a" : "2", "b" : "2", "c" : "2", "d" : "2", "e" : "2" }{ "_id" : ObjectId("5018da821781352fe25bf4d4"), "a" : "3", "b" : "3", "c" : "3", "d" : "3", "e" : "3" }{ "_id" : ObjectId("5018da891781352fe25bf4d5"), "a" : "4", "b" : "4", "c" : "4", "d" : "4", "e" : "4" }{ "_id" : ObjectId("5018da931781352fe25bf4d6"), "a" : "5", "b" : "5", "c" : "5", "d" : "5", "e" : "5" }{ "_id" : ObjectId("5018da9d1781352fe25bf4d7"), "a" : "6", "b" : "6", "c" : "6", "d" : "6", "e" : "6" }{ "_id" : ObjectId("5018daa71781352fe25bf4d8"), "a" : "7", "b" : "7", "c" : "7", "d" : "7", "e" : "7" }{ "_id" : ObjectId("5018daa71781352fe25bf4d9"), "a" : "8", "b" : "8", "c" : "8", "d" : "8", "e" : "8" }{ "_id" : ObjectId("5018daa71781352fe25bf4da"), "a" : "9", "b" : "9", "c" : "9", "d" : "9", "e" : "9" }{ "_id" : ObjectId("5018daa71781352fe25bf4db"), "a" : "10", "b" : "10", "c" : "10", "d" : "10", "e" : "10" }{ "_id" : ObjectId("5018daa71781352fe25bf4dc"), "a" : "11", "b" : "11", "c" : "11", "d" : "11", "e" : "11" }{ "_id" : ObjectId("5018daa71781352fe25bf4dd"), "a" : "12", "b" : "12", "c" : "12", "d" : "12", "e" : "12" }{ "_id" : ObjectId("5018daa71781352fe25bf4de"), "a" : "13", "b" : "13", "c" : "13", "d" : "13", "e" : "13" }{ "_id" : ObjectId("5018daa71781352fe25bf4df"), "a" : "14", "b" : "14", "c" : "14", "d" : "14", "e" : "14" }{ "_id" : ObjectId("5018daae1781352fe25bf4e0"), "a" : "15", "b" : "15", "c" : "15", "d" : "15", "e" : "15" }> db.B.find(){ "_id" : ObjectId("5018dac11781352fe25bf4e1"), "a" : "1", "b" : "1", "c" : "1"}{ "_id" : ObjectId("5018dac11781352fe25bf4e2"), "a" : "2", "b" : "2", "c" : "2"}{ "_id" : ObjectId("5018dac11781352fe25bf4e3"), "a" : "3", "b" : "3", "c" : "3"}{ "_id" : ObjectId("5018dac11781352fe25bf4e4"), "a" : "4", "b" : "4", "c" : "4"}{ "_id" : ObjectId("5018dac11781352fe25bf4e5"), "a" : "5", "b" : "5", "c" : "5"}{ "_id" : ObjectId("5018dac11781352fe25bf4e6"), "a" : "6", "b" : "6", "c" : "6"}{ "_id" : ObjectId("5018dac11781352fe25bf4e7"), "a" : "7", "b" : "7", "c" : "7"}{ "_id" : ObjectId("5018dac11781352fe25bf4e8"), "a" : "8", "b" : "8", "c" : "8"}{ "_id" : ObjectId("5018dac11781352fe25bf4e9"), "a" : "9", "b" : "9", "c" : "9"}{ "_id" : ObjectId("5018dac11781352fe25bf4ea"), "a" : "20", "b" : "20", "c" : "20" }{ "_id" : ObjectId("5018dac31781352fe25bf4eb"), "a" : "21", "b" : "21", "c" : "21" }> db.A.find().forEach(... function(x){...   var obj1 = db.B.findOne({"a":x.a,"b":x.b,"c":x.c});...   if(obj1 == undefined || obj1 == null)...   {...   var obj2 = {"a":x.a,"b":x.b}...   db.C.insert(obj2);...   }... }... )> db.C.find(){ "_id" : ObjectId("5018fccd1781352fe25bf50d"), "a" : "10", "b" : "10" }{ "_id" : ObjectId("5018fccd1781352fe25bf50e"), "a" : "11", "b" : "11" }{ "_id" : ObjectId("5018fccd1781352fe25bf50f"), "a" : "12", "b" : "12" }{ "_id" : ObjectId("5018fccd1781352fe25bf510"), "a" : "13", "b" : "13" }{ "_id" : ObjectId("5018fccd1781352fe25bf511"), "a" : "14", "b" : "14" }{ "_id" : ObjectId("5018fccd1781352fe25bf512"), "a" : "15", "b" : "15" }----------------------------------------------------实际处理脚本---------------------------------------------从集合A中查找文档并循环遍历db.A.find().forEach( function(x){   --以集合A中的每条文档的a,b,c属性做条件查找集合B   var obj1 = db.B.findOne({"a":x.a,"b":x.b,"c":x.c});   --若找不到，即在集合B中不存在，则满足要求   if(obj1 == undefined || obj1 == null)   {    --创建新的文档并保存在集合C中    var obj2 = {"a":x.a,"b":x.b}    db.C.insert(obj2);   } })*/?>