Problem:

From the documentation

http://www.mongodb.org/display/DOCS/Indexes

A unique index cannot be created on a key that has duplicate values. If you would like to create the index anyway, keeping the first document the database indexes and deleting all subsequent documents that have duplicate values, add the dropDups option.

db.things.ensureIndex(

{firstname : 1}

,

{unique : true, dropDups : true}

)

However, this appears to be broken, since its errors out with

E11000 duplicate key error index: test.fb.$name_1 dup key: { : "fred" }

Reproduce:

> db.fb.save(

{name:"fred"}

);
> db.fb.save(

{name:"fred"}

);
> db.fb.save(

{name:"fred"}

);
> db.fb.ensureIndex(

{name:1}

,

{unique:1, dropDupes:1}

);
E11000 duplicate key error index: test.fb.$name_1 dup key: { : "fred" }
> db.fb.ensureIndex(

{name:1}

,

{unique:true, dropDupes:true}

);
E11000 duplicate key error index: test.fb.$name_1 dup key: { : "fred" }

Business case:

• broken functionality