// Create a on unique index use foo db.createCollection("bar", {capped: false, autoIndexId: false} ) db.bar.getIndexes() // Should return [ ] db.bar.ensureIndex({_id:1}, {unique:false} ) // index on _id will be returned db.bar.getIndexes() db.bar.insert( {"_id" : 1} ) // The insert here will fail db.bar.insert( {"_id" : 1} ) db.dropDatabase() // now the same thing, only the index is not on the id use foo db.createCollection("bar", {capped: false, autoIndexId: false} ) db.bar.ensureIndex( {hash:1} , {unique:false} ) // index on hash will be returned db.bar.getIndexes() db.bar.insert( {"hash" : 1} ) // The insert here will succeed db.bar.insert( {"hash" : 1} ) db.dropDatabase() // now the same thing without an index use foo db.createCollection("bar", {capped: false, autoIndexId: false} ) db.bar.insert( {"_id" : 1} ) // The insert here will succeed db.bar.insert( {"_id" : 1} ) db.dropDatabase()