input: Write a python function to add a new book and fetch all books from the database using Flask, SQLAlchemy, and Marshmallow.
  assert add_book({'title': 'Book A', 'author': 'Author A'}).json == {'title': 'Book A', 'author': 'Author A'}
  assert add_book({'title': 'Book B', 'author': 'Author B'}).json == {'title': 'Book B', 'author': 'Author B'}
  assert get_books().json == [{'title': 'Book A', 'author': 'Author A'}, {'title': 'Book B', 'author': 'Author B'}]
output: from flask import Flask, request, jsonify
  from flask_sqlalchemy import SQLAlchemy
  from flask_marshmallow import Marshmallow
  
  app = Flask(__name__)
  app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///books.db'
  db = SQLAlchemy(app)
  ma = Marshmallow(app)
  
  class Book(db.Model):
      id = db.Column(db.Integer, primary_key=True)
      title = db.Column(db.String(100), nullable=False)
      author = db.Column(db.String(100), nullable=False)
  
  class BookSchema(ma.SQLAlchemyAutoSchema):
      class Meta:
          model = Book
  
  book_schema = BookSchema()
  books_schema = BookSchema(many=True)
  
  @app.route('/add_book', methods=['POST'])
  def add_book():
      title = request.json['title']
      author = request.json['author']
      new_book = Book(title=title, author=author)
      db.session.add(new_book)
      db.session.commit()
      return book_schema.jsonify(new_book)
  
  @app.route('/get_books', methods=['GET'])
  def get_books():
      all_books = Book.query.all()
      result = books_schema.dump(all_books)
      return jsonify(result)
  
  # Initialize the database
  with app.app_context():
      db.create_all()