_id is unique in a collection

---

examples/mongodb/unique_id.txt

> db.a.insert({ _id : 'a', value : 1 })
Inserted 1 record(s) in 20ms

> db.a.find()
{
  "_id": "a",
  "value": 1
}
Fetched 1 record(s) in 1ms

> db.a.insert({ _id : 'a', value : 2 })
E11000 duplicate key error index: test.a.$_id_  dup key: { : "a" }

> db.b.insert({ _id : 'a', value : 2 })
Inserted 1 record(s) in 763ms

> db.a.find()
{
  "_id": "a",
  "value": 1
}
Fetched 1 record(s) in 1ms

> db.b.find()
{
  "_id": "a",
  "value": 2
}
Fetched 1 record(s) in 1ms