In my mongo shell, I do this:

db.members.ensureIndex(

,

)

• I also try db.members.ensureIndex( {facebook_id:1}

  ,

  {sparse:true,unique:true}

  )

Then I try to insert 2 records with facebook_id = null

I have got this message:

E11000 duplicate key error index: writepub.members.$facebook_id_1 dup key: { :null }