How to retrieve a single field from MongoDB in Java

问题

I am trying to retrieve a field from a document and set it equal to a variable so when I update it later I can just do (field + newCount) or something like that.

For example my document is:

{
"username":"testusername"
"item":"Sofa"
"price":150.0
}

How do I retrieve the price field and set it equal to let's say double currentPrice; ?

The documentation is a bit ambiguous so an example would help.

Also, I am using mongoDB Java version 3.0.2

Thanks

EDIT: Apparently the question is a bit ambiguous, but I am trying to retrieve the price of the sofa that is stored in the database, and set it equal to a variable in my java code.

So, I would like to set it so that double currentPrice = 150.0;

So how can I use java code to access the price of the item and set it to a variable?

回答1:

Here is how you'd retrieve a single field from MongoDB, driver version 3.0.2 You create a query with a projection:

    Document document = collection
            .find(new BasicDBObject("username", "testusername"))
             .projection(Projections.fields(Projections.include("price"), Projections.excludeId())).first();

Then, you can extract this field as double with the following code:

    double price = document.getDouble("price");

来源：https://stackoverflow.com/questions/31141990/how-to-retrieve-a-single-field-from-mongodb-in-java