text: // @tags: [
  //     # Cannot implicitly shard accessed collections because of not being able to create unique
  //     # index using hashed shard key pattern.
  //     cannot_create_unique_index_when_using_hashed_shard_key,
  //     requires_fastcount,
  //
  //     # Investigate why this fails on embedded
  //     incompatible_with_embedded_todo_investigate,
  // ]
  
  var res;
  
  t = db.jstests_uniqueness;
  
  t.drop();
  
  // test uniqueness of _id
  
  res = t.save({_id: 3});
  assert.writeOK(res);
  
  // this should yield an error
  res = t.insert({_id: 3});
  assert.writeError(res);
  assert.eq(1, t.count());
  
  res = t.insert({_id: 4, x: 99});
  assert.writeOK(res);
  
  // this should yield an error
  res = t.update({_id: 4}, {_id: 3, x: 99});
  assert.writeError(res);
  assert(t.findOne({_id: 4}));
  
  // Check for an error message when we index and there are dups
  db.jstests_uniqueness2.drop();
  db.jstests_uniqueness2.insert({a: 3});
  db.jstests_uniqueness2.insert({a: 3});
  assert.eq(2, db.jstests_uniqueness2.count());
  res = db.jstests_uniqueness2.ensureIndex({a: 1}, true);
  assert.commandFailed(res);
  assert(res.errmsg.match(/E11000/));
  
  // Check for an error message when we index in the background and there are dups
  db.jstests_uniqueness2.drop();
  db.jstests_uniqueness2.insert({a: 3});
  db.jstests_uniqueness2.insert({a: 3});
  assert.eq(2, db.jstests_uniqueness2.count());
  res = db.jstests_uniqueness2.ensureIndex({a: 1}, {unique: true, background: true});
  assert.commandFailed(res);
  assert(res.errmsg.match(/E11000/));
  
  /* Check that if we update and remove _id, it gets added back by the DB */
  
  /* - test when object grows */
  t.drop();
  t.save({_id: 'Z'});
  t.update({}, {k: 2});
  assert.eq('Z', t.findOne()._id, "uniqueness.js problem with adding back _id");
  
  /* - test when doesn't grow */
  t.drop();
  t.save({_id: 'Z', k: 3});
  t.update({}, {k: 2});
  assert.eq('Z', t.findOne()._id, "uniqueness.js problem with adding back _id (2)");
  