How to store data that is common to two collections?

Lets consider that there is a collection called “Products” to store the product details and another collection called “Users” to store details about users.
Here, a user can post reviews to products. Now the review data is common to the product and the user.
Now I should query for the reviews for a particular product and also be able to query for reviews made by a particular user.
Also the data must be consistent and updates must be reflected in both places.
How shall store the data here? Should I create an separate collection to store reviews and use foreign keys like we do in RDBMS or is there any other way?