I have a Mongo database where in the users collection I have just 1 document.
I do a find() and a findOne() operations using the username filter.
I get what I think is an incorrect result from find() operation.
MongoDB shell version: 3.2.10
connecting to: test
Server has startup warnings:
2016-10-20T20:37:32.681-0700 I CONTROL [initandlisten]
2016-10-20T20:37:32.681-0700 I CONTROL [initandlisten] ** WARNING: /sys/kernel/mm/transparent_hugepage/enabled is 'always'.
2016-10-20T20:37:32.681-0700 I CONTROL [initandlisten] ** We suggest setting it to 'never'
2016-10-20T20:37:32.681-0700 I CONTROL [initandlisten]
2016-10-20T20:37:32.681-0700 I CONTROL [initandlisten] ** WARNING: /sys/kernel/mm/transparent_hugepage/defrag is 'always'.
2016-10-20T20:37:32.681-0700 I CONTROL [initandlisten] ** We suggest setting it to 'never'
2016-10-20T20:37:32.681-0700 I CONTROL [initandlisten]
> use lab2
switched to db lab2
> db.users.find()
{ "_id" : ObjectId("5807ac0765f24dd0660e4332"), "username" : "avtrulzz", "fname" : "Abc", "lname" : "Def", "email" : "test@yahoo.co.in", "password" : "rootuser", "mobile" : NumberLong(1234567890) }
> db.users.findOne()
{
"_id" : ObjectId("5807ac0765f24dd0660e4332"),
"username" : "avtrulzz",
"fname" : "Abc",
"lname" : "Def",
"email" : "test@yahoo.co.in",
"password" : "rootuser",
"mobile" : NumberLong(1234567890)
}
> if (db.users.find({username : "noSuchUsername"})) {
... print ("Username exists");
... } else {
... print ("User does not exist"); }
Username exists
> if (db.users.findOne({username : "noSuchUsername"})) { print ("Username exists"); } else { print ("User does not exist"); }
User does not exist
> if (db.users.findOne({username : "avtrulzz"})) { print ("Username exists"); } else { print ("User does not exist"); }
Username exists
See the find() operation is returning user exists which is not true. findOne() is behaving correctly.
解决方案
First of all, basic difference between findOne() and find():
findOne() - if query matches, first document is returned, otherwise null.
find() - nomatter number of documents matched, a cursor is returned, never null.
So when put in an if condition, findOne() can convert to false when it doesn't match any document. As find returns a cursor object and never returns null, will convert to true when put in an if condition.
find and findOne() return the following for empty collection :
