multiple choice question bank database structure in Mongo

[come_again]

I am making a multiple choice question bank where users can create tests from a much larger set of questions (e.g., create a test of 50 questions out of 500 lets say). My question is what is the best way to store this information? I would like to be able to search used, unused, correct, incorrect etc. efficiently. Using SQL is "simpler" where I have a test table storing each answer (along with answer selected, correct/incorrect etc). In SQL, I had almost everything stored as a number (e.g., question_id, answer_id selected which related to a question table, answer table) but in mongo I feel as if I am storing everything as a string and this would slow it down significantly. Should I try and mimic what I did in SQL? How should I store multiple answer choices (which can vary in number) as well as the number of times that answer choice has been selected? This is what I am thinking of:

var SubjectsSchema = new Schema ({
    subject: { type: String }
});

var TopicsSchema = new Schema ({
    topics: { type: String }
});

var QuestionSchema = new Schema ({
    author: { type: String },
    title: { type: String },
    subject: { type: Number },
    topic: { type: Number },
    question: { type: String }, 
    answers: { },
    explanation: { type: String } 
});

var TestSchema = new Schema ({ 
    user: { type: String },
    test_number: { type: Number },
    completed: { type: Boolean },
    date: { },
    subjects: { },
    topics: { },
    noq: { type: Number }, // number of questions
    questions: { }
});