City:
    -
        _id: MongoId(553a2b522fdb44fd198b458e)
        name: Reading
    -
        _id: MongoId(553a2b522fdb44fd198b457e)
        name: London

User:
    -
        _id: "MongoId(53f31b5ad1703ad4398b4567)"
        name: Benny
        city: MongoDBRef(City,553a2b522fdb44fd198b458e)

City:
    -
        _id: MongoId(553a2b522fdb44fd198b458e)
        name: Reading
    -
        _id: MongoId(553a2b522fdb44fd198b457e)
        name: London

User:
    -
        _id: "MongoId(53f31b5ad1703ad4398b4567)"
        name: Benny
        city: MongoDBRef(City, 553a2b522fdb44fd198b458e)

Introduce invalid ID error by adding a space after comma
Introduce invalid ID error by adding a space after comma
