MongoDB show duplicate key is null?

Question

MongoDB version 3.6

I got the duplicate key message when I executed this. What's the actual problem?

mongos> db.setup.findAndModify({
... query : {"_id" : ObjectId("5b3c7abbea95705803084cad")},
... update : {$set : { "test" : "xxxx" }},
... upsert : true
... })
2018年07月06日T10:13:22.749+0000 E QUERY [thread1] Error: findAndModifyFailed failed: {
 "ok" : 0,
 "errmsg" : "E11000 duplicate key error collection: testdb.setup index: name dup key: { : null }",
 "code" : 11000,
 "codeName" : "DuplicateKey",
 "$clusterTime" : {
 "clusterTime" : Timestamp(1530872002, 603),
 "signature" : {
 "hash" : BinData(0,"AAAAAAAAAAAAAAAAAAAAAAAAAAA="),
 "keyId" : NumberLong(0)
 }
 },
 "operationTime" : Timestamp(1530872002, 602)
} :
_getErrorWithCode@src/mongo/shell/utils.js:25:13
DBCollection.prototype.findAndModify@src/mongo/shell/collection.js:724:1
@(shell):1:1
mongos>

Answer 1

This is the expected case if you have defined a unique index which is non-sparse: documents missing a value for the field with the unique index will have an indexed value of null. This means that only a single document in the collection can be missing the unique field.

The duplicate key message includes the index name and key violation:

"errmsg" : "E11000 duplicate key error collection: testdb.setup index: name dup key: { : null }",

In your example, the collection setup in database testdb has a unique index on the name field. The attempted upsert failed because the name field was missing and there was already a document in this collection with a missing or null value for name.

If you want to use a unique index without requiring the field to be present you have a few options:

• Drop and recreate the unique index with the sparse property:

  db.setup.createIndex(
   {name: 1},
   {unique:true, sparse:true}
  )
  

• Drop and recreate the unique index using a partial filter expression (which allows further criteria if needed):

  db.setup.createIndex(
   { name: 1},
   { unique:true, partialFilterExpression: {name: {$exists:true }}}
  )
  

• tao hiko: Thanks, @Stennie, it's my bad to not see "name" field in error message.
• Miguel: Hi. This is not working for me. It still complains about duplicates in the unique field. Can "duplicate documents" contains the field {...,"name":null,...} or they just should not contain that field?
• Stennie: @Miguel I suggest posting a new question with relevant details for your environment (MongoDB server version, driver version, error message, ...).
• Miguel: @Stennie, I just posted the question here. Thanks

Answer 2

Delete the collection from the DB and then hit he API , it will work. There is no mistake in code.

• Hannah Vernon: it would be great if you'd register your account and add some details around your answer. Maybe a pointer to the documentation that shows how to delete the collection or something.

Answer 3

I found other indexes on that collection have been set "unique" to be "true" (exclude _id) then it shows error.