mongo/jstests/core/update_setOnInsert.js

// This tests that $setOnInsert works and allow setting the _id
t = db.update_setOnInsert;
var res;

function dotest( useIndex ) {
    t.drop();
    if ( useIndex ) {
        t.ensureIndex( { a : 1 } );
    }

    t.update( { _id: 5 }, { $inc : { x: 2 }, $setOnInsert : { a : 3 } }, true );
    assert.docEq( { _id : 5, a: 3, x : 2 }, t.findOne() );

    t.update( { _id: 5 }, { $set : { a : 4 } }, true );

    t.update( { _id: 5 }, { $inc : { x: 2 }, $setOnInsert : { a : 3 } }, true );
    assert.docEq( { _id : 5, a: 4, x : 4 }, t.findOne() );
}

dotest( false );
dotest( true );

// Cases for SERVER-9958 -- Allow _id $setOnInsert during insert (if upsert:true, and not doc found)
t.drop();

res = t.update( {_id: 1} , { $setOnInsert: { "_id.a": new Date() } } , true );
assert.writeError(res, "$setOnInsert _id.a worked" );

res = t.update( {"_id.a": 4} , { $setOnInsert: { "_id.b": 1 } } , true );
assert.writeError(res, "$setOnInsert _id.a/b worked" );

res = t.update( {"_id.a": 4} , { $setOnInsert: { "_id": {a:4, b:1} } } , true );
assert.writeError(res, "$setOnInsert _id.a/a+b worked" );