Not Able to update dynamic fields

[Deepesh naini]

I have a model with the following structure
{
firstName: "deepesh",
lastName: "naini",
email: "theinvin...@gmail.com"
question: {
5611662e337ae3605046f0d5: false,
5611662e337ae3605046f0d7: false
},
}

Now what I want is based on a condition I want to update the boolean values to 
{
firstName: "deepesh",
lastName: "naini",
email: "theinvin...@gmail.com"
question: {
5611662e337ae3605046f0d5: true,
5611662e337ae3605046f0d7: true
},
}

But when I am calling the save function on my model after changing the boolean values to true it's not getting updated in DB.